Step 1

Begin by making the custard portion for the diplomat cream so it has time to cool. Place the egg yolks, granulated sugar and butter in a small saucepan over low heat and whisk continuously for about 5 minutes.

Step 2

The mixture will look chunky before it melts. After it melts, continue cooking it until it thickens enough to coat the back of a spoon and resembles lemon curd. Quickly remove it from the heat, pour it into a heat safe container and cover it with a lid. Set it aside to cool completely to room temperature.

Step 3

Preheat the oven to 350F/177C convection (325F/163C conventional) and grease four 8-inch pans.

Step 4

Place the egg whites in a large bowl and use an electric mixer to beat the egg whites at full speed until they're frothy and have reached the top of the bowl, add in the cup of sugar and beat until the meringue has stiff peaks.

Step 5

Pour in the egg yolks and mix just until they're fully combined. With the mixer on low-medium speed, add in the flour and baking soda, 1-2 Tbsp at a time. Mix just until the flour is incorporated.

Step 6

Distribute the batter evenly among the four pans and bake for about 17 minutes, until the cake is golden and separates itself from the edge of the pan. It will also spring back when you press on it gently. Flip the cakes over onto a tea towel and allow them to cool completely.

Step 7

Meanwhile, dissolve the 1/4 cup of granulated sugar in the coffee and allow it to cool while the cakes bake.

Step 8

At this point, dissolve the gelatin in water and the espresso powder in water and set aside.

Step 9

Use an electric mixer with a large bowl to beat together the heavy whipping cream with the powdered sugar until stiff peaks form. Add in the cooled custard mixture, the dissolved coffee and the dissolved gelatin and beat the cream at medium speed, just until it is fully combined, don't over-mix.

Step 10

Fold in the Oreo chunks.

Step 11

If the cake layers are a little uneven, use the largest one on the bottom and ALWAYS flip them upside down. The part of the cake that was on the bottom of the pan should always be facing upright when they're stacked.

Step 12

Use 1 cup of the coffee to soak the first layer of cake (you can use a squeeze bottle, a sippy cup or just a spoon). Pour it slowly, a little at a time, letting it soak in and then do the rest. Make sure to use a full cup of coffee per layer of cake.

Step 13

Use about 1/4 of the cream and spread it evenly on top of the first layer. Place the next layer on top and soak it with the coffee. Repeat this step until all the layers are used.

Step 14

Refrigerate the cake for at least 2 hours before using the rest of the diplomat cream to frost the outside of the cake. DO NOT refrigerate the diplomat cream separately because the gelatin will set.

Step 15

When the cake is finished, refrigerate it for at least another 2 hours before slicing and serving. Coffee Oreo Sponge Cake is best served cold or slightly chilled.