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Key lime pie is characterized by a crunchy, buttery graham cracker crust filled with a creamy, tart-and-sweet custard of tiny, sour Key limes. Cool and refreshing, it's a summer staple. When it came to turning the pie into a cake, I did some research and found many lime-flavored cakes or custard-filled cupcakes flavored with simple vanilla or cream cheese-based icings. None of these had the crumbly bite of the graham crust which, for me, is of utmost importance.
I started with a fluffy yellow cake that, with the addition of buttermilk, would play up the acidity of the lime curd. As for the custard, it's a standard stirred one: start by beating egg yolks, which will act as the thickening agent, with granulated sugar. This mixture should be whisked aggressively until it turns thick and very pale yellow—a method that "melts" the butter into the yolks. Sugar helps prevent eggs from thickening and setting too quickly, which will guarantee smooth results.
Lime zest and juice are added, then cooked over medium heat. Even though acid also prevents egg proteins from binding and setting, you do need to whisk constantly to avoid scrambling. The curd should only be cooked until thickened; look for a pudding-like consistency and immediately remove from heat. Carry-over cooking—when the pot is still hot and continues to cook to the mixture—should be taken into account; it is best to transfer the curd to a bowl and allowed to cool there. Prevent a skin by placing a piece of plastic wrap directly onto the surface of the curd, or cool quickly by placing the curd in an ice bath, whisking until completely cooled.
The curd is the base for this cake's icing. To stabilize it, I add dissolved plain gelatin, a couple ounces of cream cheese, confectioners' sugar, and whipped cream for creamy consistency.
Now, I said that I couldn't do without the signature graham cracker crunch, so my solution was to make a crust of graham cracker crumbs, melted butter, and light brown sugar, then break it up into pieces and crumble it over the cake.
This is a perfect summer cake; light and fluffy and coated with a tangy Key lime icing. A layer of buttery cookie crumbs adds textural contrast.