Special equipment: 2 round 9-inch cake pans, 1 metal (preferably offset) spatula, 1 instant-read thermometer.
Preheat the oven to 350 F. Put parchment paper in the bottom of the two cake pans and use nonstick spray (or a little butter) to thoroughly grease the sides and bottoms. Set aside.
Make the batter for the cake: In the bowl of the 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.
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. Mix in the mayonnaise, gently whisk in the hot water and then fold in the egg/sugar mixture until smooth.
Mix until all of the ingredients are thoroughly combined. Pour half of the cake batter into each of the prepared pans. Place the pans in the center of the oven and bake for 20 to 25 minutes. Test the cakes to make sure they are cooked in the center. Remove from the oven and set aside to cool.
For the frosting: Make a stovetop water bath by placing a large (somewhat deep) skillet half-filled with water on the stove. Heat the water until it simmers then shut it off.
Meanwhile, put the egg whites, sugar, cream of tartar, 5 tablespoons water and corn syrup in a clean metal bowl. Whisk to blend. 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. Holding a thermometer and whisking at the same time can be difficult. When the whites reach 140 degrees F, remove the bowl from the water bath. Stir in the vanilla extract and whisk for 1 to 2 additional minutes to cool it off a little. Cover with plastic wrap until ready to frost the cake.
Assemble the cake: Gently turn the cake rounds out onto a flat surface. Finely grate some chocolate onto the first layer of the cake. Then, use about 1/3 of the total frosting on top of the first layer, letting some of it spill over the top to coat the sides. Place the other layer squarely on top of the first and use the remaining frosting to coat the top and sides of the cake. Grate more chocolate over the cake. This chocolate will create a nice little texture to compliment the soft cake.