Key Lime Custard Slice


Last week I extolled the virtues of blood oranges. I love that it is citrus season and this week I’ve got another citrusy offering with key lime custard slices. 

A custard slice is commonly found in Australian bakeries and is usually vanilla custard sandwiched between pastry and iced with a vanilla royal icing.

If you watch the Great British Bake Off- and, if you don’t, you should!- custard slice was a challenge in this most recent season during the finale. I am often inspired to try my hand at the challenges presented to the contestants who are an incredibly talented group of people. I am sure I could not withstand the pressures they are under, particularly time and weather.

The light pastry and creamy custard really got my attention. And, I wanted to make something different with key limes than a traditional key lime pie. (On a side note, if you ever have a chance to have a slice of key lime pie in Key West, do not hesitate. It is the best of any other version of it anywhere.)

This key lime custard slice is rich with the fresh tang of key lime, sandwiched between crisp light puff pastry and topped with a key lime icing and a few crumbles of graham cracker as homage to the pie.

This recipe owes most of its process to the wonderful recipe by Dave Friday, Caramel Latte Custard Slice, which is incredibly delicious! 

Dave gives the recipe for making rough puff pastry, which I haven’t tried yet, still wedded to the much easier and predictable frozen puff pastry sheets. So if you choose to try his version, I applaud you, but if using frozen, roll out the two puff pastry sheets that come in the box until they are about 9” X 9” square and very thin, about 1/16” thick. 

This involves 4 baking sheets but if you only have two, then bake them one at a time. Place one pastry sheet onto a baking sheet lined with parchment paper, prick with a fork.

Top with another piece of parchment paper. Place a second baking sheet on top of the parchment enclosed pastry. Repeat with the second pastry sheet.

Bake them both at 450 for 20-25 minutes, until golden brown.

While those are baking, get your custard going. I use a basic custard recipe that works for ice cream, pudding and adapting to any kind of custard filling required. It begins with whisking egg yolks and sugar together and adding in scalded half and half.

This custard needs to be pretty sturdy without being rubbery, because it will be sliced and you do not want it oozing out everywhere, so I added cornstarch to the mixture, along with the zest of four key limes. 

This recipe calls for key lime zest from fresh limes. Sometimes they aren’t always available but you can substitute regular lime zest here.

Whisk in the scalded milk a little at time to make sure you don’t scramble the eggs. Then return the entire mixture to the pan.

In order to help the custard firm up, a little gelatin is needed. Gelatin is sold in different forms- clear sheets or powder. I believe powder form is the most common in the US, and it is all I could find, so the proportions in the recipe below are based on using powdered gelatin.

This requires “melting” the gelatin into a liquid form, combining a bit of the gelatin powder with boiling water. Do this, stirring to fully melt the gelatin and set aside.

Also, you will want to have some key lime juice and butter at the ready. Here, again, you can work really hard and juice A LOT of little key limes, or you can buy the Nellie and Joe’s classic key lime juice. I have found no difference between juicing my own and this juice and it is closest in flavor to that found in Key West lime dishes.

Begin heating your custard mixture over a medium high heat, whisking constantly until it thicken. Remove from the heat and whisk in the lime juice and the butter. Once both are fully incorporated, add the gelatin and stir well. 

Strain that through a mesh sieve into a clean bowl and place a piece of plastic wrap directly onto the custard to prevent a skin from forming. Just set that aside at room temperature. You want it to cool slightly but not firm up quite yet.

Meanwhile, your pastry should be ready.

Just after removing them from the oven, sprinkle with some powdered sugar. Once they are cool enough to handle, you want to cut them. This is an easy process, although it takes a light hand because the pastry is thin and fragile.

I love this pan with a removable bottom for recipes like this.

You can, of course, use any 8” x 8” square baking pan that you have, but I recommend these false bottom pans if you are in the market for a new baking pan. They make baking bar cookies and cakes really stress free, as you can release your baking without having to scrape down the sides, risking tears in your cake or cookies.

Use the removable bottom of the pan, as I did, or just the bottom of your baking pan as a guide to cut the pastry. Gently, cut around the perimeter of the pan bottom to create two equal squares that fits your pan. Additionally, to prevent too tight a squeeze, I sliced off about 1/8” from the top and one side of each of the pastry sheets. 

Line your pan with 2 pieces of plastic wrap set perpendicularly to each other. Make sure to leave some hanging on each side to grab if need when it comes time to remove. This is especially important if you do not have a removable bottom pan.

Place one piece of pastry into the bottom of the lined pan.

Pour the custard on top and spread evenly across the pastry.

Top with the second pastry square. Cover the whole pan with a piece of loose plastic wrap and refrigerate for at least 2 hours. 

Once the custard has fully firmed up, it is time to remove the entire dessert from the pan. Carefully push the bottom up and release the dessert from the pan if you have a removable bottom. Otherwise, use the plastic wrap “handles” to pull the dessert straight up and out of the pan.

Make a royal icing with powdered sugar and key lime juice. You want this to be fairly thick so it doesn’t run down the sides of the pastry.

Spread the icing gently and evenly across the top of the pastry to cover it completely. Crush up a graham cracker and sprinkle crumbs on top of the icing, along with some zest of key lime.

At this point you might want to refrigerate it for a half an hour to make cutting it easier. 

Now, this is the most difficult and, admittedly, messy part. It is important, when cutting the pastry, that you do not press down with the knife but keep a continuous back and forth motion, letting the knife do the work. Also helpful to maintain the nice layered look, is to wipe the knife between cuts with a paper towel or kitchen towel.

To cut the pastry into the traditional slice, cut into 1” wide strips in one direction. Then cut each strip in half. so that you have pieces that are about 1” x 4”.

Place the slices on a pretty serving platter and enjoy! 

The tart of the key limes and the sweet of the pastry and icing balance perfectly. They are sticky and messy and just plain delicious!

Key Lime Custard Slice

Key lime custard slice is rich with the fresh tang of key lime, sandwiched between crisp light puff pastry and topped with a key lime icing and a few crumbles of graham cracker as homage to the pie.
Servings 16 servings
Prep Time 20 mins
Cook Time 30 mins
Refrigeration Time 2 hrs


  • 4 baking sheets
  • parchement paper
  • saucepan
  • 2 mixing bowls
  • 8" square baking pan


Puff pastry

  • 2 frozen puff pastry sheets
  • powder sugar

Key Lime Custard

  • 1/2 cup sugar
  • 4 egg yolks
  • 1/8 cup cornstarch
  • 2 tbsp key lime zest *see note
  • 1 cup half & half
  • 2 tsp powdered gelatin non-flavored
  • 1/8 cup boiling water
  • 1/2 cup key lime juice **see note
  • 2 tbsp butter

Key Lime Icing

  • 1 1/2 cup powdered sugar
  • 2 Tbsp + 2 tsp key lime juice
  • 1 graham cracker crushed
  • key lime zest


Puff Pastry

  • Preheat oven to 450°.
  • Roll each sheet into a thin 9" x 9" square, about 1/16th" thick.
  • Place each pastry sheet onto parchment lined baking sheet. Pierce all over with a fork.
  • Top with another piece of parchement paper and then a second baking sheet.
  • Bake for 20-25 minutes until golden brown.
  • Sprinkle with powdered sugar immediately after removing from oven. Let cool.

Key Lime Custard

  • In a mixing bowl, whisk together sugar, egg yolk, cornstarch, and lime zest until pale.
  • Heat half & half until scalded- small bubbles formed around the perimeter of the pan. Do not boil.
  • Whisking constantly, slowly add half & half to egg mixture.
  • Return mixture to the pan.
  • Prepare gelatine by combining gelatin powder with boiling water. Stir until gelatin is dissolved fully. Set aside.
  • Over medium high heat, begin cooking the custard mixture, whisking constantly until thick. It should be able to thickly cover the back of a wooden spoon.
  • Remove from heat and stir in the lime juice, whisking constantly until fully incorporated. Do not stop whisking until all the juice is full blended in or you risk the mixture curdling.
  • Add in the butter and a pinch of salt. Stir to combine well and all butter has melted in.
  • Add the gelatin mixture, stirring well to fully combine.
  • Strain the mixture through a mesh sieve into a clean bowl.*** Place a piece of plastic wrap directly onto the surface of the custard and set aside at room temperature.


  • Using the bottom of your baking pan cut each pastry sheet to fit the pan. Be gentle as the pastry is light and fragile and may crack easily. However, the pastry cuts easily with very little effort. Cut 1/8" off of the top and one side of the newly cut sqaure pastry sheet to make fitting easier.
  • Place two long pieces of plastic wrap into your pan perpendicular to one another, leaving extra to overhand on each side.
  • Place one pastry sheet, sugar side up, into the bottom of the pan.
  • SPread teh custard evenly over the top of the pastry sheet.
  • Place the second pastry sheet, sugar side down, on top of the custard, not pressing down but making contact with the custard.
  • Cover loosely with plastic wrap and refrigerate for at least 2 hours.
  • Make the icing by combining the powdered suagr and the key lime juice in a bowl. It should be fairly thick.
  • Remove the pan from the refrigerator and remove the entire dessert from the pan, either by removing the bottom of a false-bottom pan or by using the overhanging plastic wrap as handles to pull the dessert straight up and out of the pan. Carefully remove the plastic wrap from underneath the dessert.
  • Spread the icing evenly across the top of the pastry. Sprinkle with graham cracker crumbs and zest.
  • Refrigerate if necessary to maintain firmness.
  • To cut the dessert into the slices, use a serrated or other sharp knife to gently cut 1" strips across the square. Do not push down but keep a continuous back and forth motion. Wipe the knife between each slice to maintain clean layers.
  • Cut each strip in half so that you end with 16 1" x 4" slices.


*regular lime zest can be substituted where fresh key limes are unavailable
** Fresh squeezed juice from key limes or bottled key lime juice like Nellie and Joe’s Famous Key Lime Juice
*** The lime zest will be strained out. If you like the appearance of the zest in the custard, simply zest a bit more into the strained custard.
Course: Dessert
Cuisine: American
Keyword: custard, key lime, puff pastry

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