In a medium saucepan add the sugar and cornstarch together and whisk until evenly combined.
3/4 cup granulated sugar, 1/4 cup cornstarch
Turn the stove to medium heat and slowly add in the egg yolk and half and half mixture. Bring to a boil, whisking constantly.
4 large egg yolks, 2 cups half and half
Once boiling, let the mixture bubble for one minute, then immediately remove the saucepan from the heat and whisk in the butter, key lime zest, and key lime juice. Keep whisking until the ingredients are completely incorporated into the mixture.
3 TBS butter, 2 TBS key lime zest, 1/2 cup key lime juice
Line an 8x8 square baking dish with plastic wrap, including the sides.
Line the bottom of the 8×8 pan with a single layer of graham crackers. It’s easiest to line them three across and then break a few apart to fill in the ends.
15 graham crackers
After the key lime curd has cooled for 10 minutes, spoon roughly 3/4 cup curd over the top of the crackers to create an even layer of key lime curd.
Repeat with three more layers of graham crackers and custard, add the last layer of graham crackers over the fourth layer of custard. Then cover the pan and place in the freezer for at least eight hours or overnight.
Just before serving, remove the cake from the freezer and using the plastic wrap, carefully remove the cake from the pan. Allow to thaw for an hour. t
Top with whipped cream. Slice and garnish with key lime zest and key lime wedges if desired for seving.
Whipped Cream
Add the whipping cream and powdered sugar to a medium bowl and mix with a hand mixer on medium high speed (or in a stand mixer) until soft peaks form.
1 cup heavy whipping cream, 1/4 cup powdered sugar
Notes
Play around with the flavors by making a lemon and honey curd instead of key lime. Or add a layer of sweetened condensed milk on the graham crackers before the curd for added sweetness. Store in an airtight container in the fridge for up to three days. Store in the freezer for up to one month. Allow the cake to thaw for at least an hour before eating if you freeze.