1. Heat the oven to 350°F. Spray a metal 9- by-13 inch sheet pan (or two 9-inch round cake pans with nonstick cooking spray. (If using round cake pans, line the bottom of each pan with parchment paper.)
2. In a medium bowl, sift together the flour, cocoa powder, baking powder, baking soda, and salt.
3. In the bowl of a stand mixer fitted with the paddle attachment, beat the sugar and eggs together at medium-high speed until light and fluffy, 3 to 4 minutes. Beat in the mayonnaise and the vanilla until smooth.
4. Lower the mixer speed to its lowest speed and beat in 1/2 the dry ingredients just until combined. Stop the mixer and scrape down the sides of the bowl.
5. In a small bowl, mix together the espresso powder with the boiling water. Add 1/2 of the espresso mixture to the batter and beat on low speed just until the batter is smooth, about 5 to 10 seconds. Add the remaining dry ingredients and beat just until combined. Beat in the remaining espresso and beat just until smooth. The batter will be somewhat thin.
6. Scrape the batter into the prepared pan (s) and bake 22 to 25 minutes until a wooden skewer inserted in the middle of the cake comes out clean. Transfer the pan to a wire rack and let cool completely.
7. For the frosting: In a large, microwave-safe bowl, combine the semisweet and milk chocolate chips with the butter. Heat on high power for 1 minute. Stir together until the butter and chocolates are completely melted and smooth. If not completely melted after 1 minute, heat again in 15-second increments, stirring until smooth. Use a large balloon whisk or a wooden spoon and stir in the sour cream. Beat in add 2 cups [xx g] confectioners’ sugar, just until smooth and spreadable. If a thicker, sweeter frosting is desired, beat in an additional cup of sugar.
8. Spread the top of the cooled cake with the Melted Chocolate Bar Frosting. Cut into squares and serve. Cover pan tightly with plastic wrap or foil, and store at room temperature. The cake will stay fresh for about 2 days. (For layer cake: place one layer on a cake stand and frost the top with about 1 cup frosting. Top with second layer and spread the remaining frosting on the top and around the sides of the cake.)