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1
Heat the oven to 350u00b0F and position a rack to the center. Spray 2 9 x 13-inch cake pans with nonstick spray and set aside. Whip the egg whites with the salt until stiff. Add the sifted confectioners sugar, 1 tablespoon at a time. Fold in the cocoa until fully incorporated.
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2
In a separate bowl, beat the egg yolks until thick. Then fold the yolks into the cocoa mixture. Add the vanilla extract.
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3
Divide the cake batter between the 2 prepared cake pans, spreading the batter as evenly as possible. Bake for 20 minutes. When the cake layers are finished, they will be a light golden brown and will have pulled away from the edges of the pan. Cool for 10 minutes in the pans, then transfer to wire racks to cool fully before filling.
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4
For the filling, combine the heavy cream, sugar, cocoa, instant espresso and vanilla, mixing until everything is fully incorporated. Then mix on high until the filling thickens and is smooth and fluffy.
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5
Spread the filling onto the top of one cake layer, then top with the second layer. Chill for 1 hour.
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6
For the glaze, melt the chocolate and butter together, stirring to combine. Mix in the boiling water, confectioners sugar, salt and vanilla extract on low speed. And then once combined, turn the mixer up to medium until the glaze is smooth and glossy.
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7
To finish assembly, trim the edges off the filled cake, then smooth the glaze on top of the cake. Chill to set the glaze.
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8
Before you serve the cake, remove it from the fridge about an hour ahead of time. Slice the cake into small squares.
