MAKE CAKE: Preheat oven to 350°F. Grease and flour 2 (9-inch) round cake bans and line bottoms with parchment paper.
WHISK coffee and cocoa powder together in a 2-cup glass measuring cup until completely blended. Let stand until cooled.
SIFT together flour, baking soda, salt and baking powder in a large bowl; set aside. In another bowl, combine brown sugar and eggs and beat with a mixer at high speed until light and fluffy, about 3 minutes, stopping to scrape sides of bowl. Add mayonnaise and beat at low speed until well combined. Gradually add flour mixture alternately with coffee mixture, beginning and ending with flour mixture, beating just until combined after each addition.
DIVIDE batter between prepared pans. Bake for 28 to 35 minutes or until a toothpick inserted in center comes out clean. Let cool in pan for 10 minutes. Remove from pans onto wire racks and let cool completely. Level cake layers, if desired; remove and discard parchment.
MAKE FROSTING: Melt chocolate and butter in the top of a double boiler, stirring occasionally until smooth. Remove from heat; let cool for 5 minutes.
BEAT confectioners’ sugar, sour cream and vanilla in a large bowl, with a mixer on low speed, until smooth and well combined, stopping to scrape bowl as needed. Gradually beat in chocolate mixture until well combined, stopping to scrape bowl. Increase speed to medium and beat until fluffy, about 2 minutes. Use immediately.
ASSEMBLE CAKE: Spread 1 1/2 cups Creamy Chocolate Frosting between layers. Spread remaining frosting on top and sides of cake.