Preheat the oven to 350 degrees F. Thoroughly grease the bottoms and sides of three 8-inch round cake pans with the butter.
Make the cake batter. In the bowl of a stand mixer fitted with the whisk attachment, beat the eggs and sugar on high speed until light, fluffy, and lemon-colored... about 5 to 8 minutes.
In a medium bowl, whisk together the mayonnaise, cocoa powder, coffee granules, baking soda, baking powder, and salt.
Remove the bowl from the mixer and whisk the mayonnaise mixture into the egg mixture until smooth. Stir in the flour until well combined.
Gently stir in 1 1/2 cups hot water. Mix until all of the ingredients are thorougly combined.
Bake the cake layers. Divide the batter into thirds, pouring into the prepared cake pans. Bake until a toothpick inserted in the center comes out clean, 25-30 minutes. Allow the layers to cool completely before frosting.
Make the frosting. Make a stovetop water bath by filling a large skillet halfway with water and heating the water until it simmers. In a clean medium-size heatproof bowl, whisk together the egg whites, sugar, cream of tartar, 1 tablespoon of water, and the corn syrup.
Set the bowl in the water bath and wisk the mnimxture constantly (with the heat off) until it fluffs up and reaches a temperature of 140 degrees F. You can also use a mixer on low speed for this.
Remove the bowl from the water bath, stir in the vanilla extract, and whisk for another 1-2 minutes to cool it off a little.
Assemble and frost the cake! Top with chocolate curls or grated chocolate and enjoy.