• Preheat oven to 350 degrees F. Grease two 9-inch round cake pans. Line bottoms with parchment circles if desired, then grease again and dust with flour.
• In a large saucepan, boil the water. Remove from heat and add the espresso powder and cocoa powder. Stir until smooth and set aside.
• In the bowl of a stand mixer, beat the eggs, sugar and brown sugar for a full 7 minutes or until thick and light. Add the mayonnaise and vanilla and beat on medium until combined.
• In a second bowl, thoroughly mix together the flour, baking powder, baking soda, and salt.. Add to the egg mixture and stir until mixed.
• Pour in the cocoa mixture and stir well. Batter will be thin.
• Pour into prepared pans and bake on center rack for 30 minutes. Remove from oven and let cool on a wire rack for 10 minutes. Invert cakes on a cooling rack, peel away the parchment (if you used it), and let cool completely.

Sour Cream Chocolate Frosting

• Be sure to bring all of the ingredients to room temperature. If cocoa powder and sugar are lumpy, it doesn't hurt to sift them.
• Beat the butter until light and creamy, then add the cocoa powder and beat until mixed. Gradually add the confectioners’ sugar (and salt, if using) and sour cream and beat until smooth. Beat in the vanilla.