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Special Equipment: 2 round cake pans, 9 inches in diameter; 1 metal (preferably offset) spatula for frosting the cake; 1 instant-read thermometer
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Preheat the oven to 350 degrees F. Put parchment paper in the bottom of the 2 cake pans and use nonstick spray (or a little butter) to thoroughly grease the sides and bottoms.
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Set aside.
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Make the batter for the cake: In the bowl of an electric mixer fitted with the whisk attachment, combine and beat the eggs and sugar on high speed until light, fluffy and lemony-colored, 5 to 8 minutes.
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Meanwhile, in a medium bowl, whisk together the flour, baking powder, baking soda, cocoa powder (sifted over the other ingredients), coffee (if using) and salt.
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Mix in the mayonnaise, gently whisk in the hot water and then fold in the egg/sugar mixer from the machine until smooth.
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Mix until all of the ingredients are thoroughly combined.
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Pour half of the cake batter into each of the prepared pans.
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Place the pans in the center of the oven and bake for 20 to 25 minutes.
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Test the cakes to make sure they are cooked in the center.
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Remove from the oven and set aside to cool.
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Make the frosting: Make a stovetop water bath by placing a large (somewhat deep) skillet half-filled with water on the stove.
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Heat the water until it simmers, and then shut it off.
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Meanwhile, put the egg whites, sugar, cream of tartar, 5 tablespoons water and corn syrup in a clean metal bowl.
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Whisk to blend.
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Place the bowl in the water bath and (with the heat off) whisk the whites by hand or with an electric hand mixer until they fluff up and reach a temperature of 140 degrees F. Whisking the whites constantly is important so just test the temperature of the whites from time to time with the thermometer.
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Holding a thermometer and whisking at the same time can be difficult.
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When the whites reach 140 degrees F, remove the bowl from the water bath.
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Stir in the vanilla extract and whisk for 1 to 2 additional minutes to cool it off a little.
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Cover with plastic wrap until ready to frost the cake.
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Assemble the cake: Gently turn the cake rounds out onto a flat surface.
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Finely grate some chocolate onto the first layer of the cake.
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Then, use about one-third of the total frosting on top of the first layer, letting some of it spill over the top to coat the sides.
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Place the other layer squarely on top of the first and use the remaining frosting to coat the top and sides of the cake.
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Grate more chocolate over the cake.
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This chocolate will create a nice little texture to compliment the soft cake.