Chocolate Mayonnaise Cake

Cake
4 Preheat oven to 350°F
5 Grease three 8 -inch cake pans and dust with cocoa.
8 Melt the chocolate in the microwave just until softened and you can stir it smooth - about 1 minute.
9 Combine melted chocolate and cocoa in a bowl. Whisk in boiling coffee (or water) and stir until the mixture is smooth. Set aside.
10 Sift the flour, baking soda, baking powder, and salt into a bowl. Set aside.
13 With an electric mixer on medium-high speed beat the sugar and the mayonnaise until blended. 
14 Add eggs one at a time, beating well after each. Add the egg yolk.
15 Beat in the vanilla.
16 Change to low speed. Add the dry ingredients in four additions, alternating with the chocolate mixture.
17 Beat well after each addition.
20 Divide cake batter evenly among the cake pans.
21 Bake about [timer minutes=0]30 minutes[/timer] or until a cake tester comes out clean.
22 Cool cake for [timer minutes=0]10 minutes[/timer] in pans.
23 Loosen cakes from sides of pans, turn out, and cool completely before frosting.
Frosting
26 In a bowl of stand mixer fitted with paddle attachment beat the butter on medium speed about [timer minutes=0]6 minutes[/timer] or until pale and fluffy.
27 Sift together the confectioners' sugar and cocoa powder.
28 Reduce mixer speed to low.
29 Add the confectioners' sugar mixture, beating until well blended.
30 Continue beating on low and add the coffee, sour cream, vanilla, and salt.
31 Beat [timer minutes=0]1 minute[/timer].
32 Increase the speed to medium and whip for [timer minutes=0]2 minutes[/timer].
33 Add the  melted chocolate and beat until smooth, about [timer minutes=0]2 minutes more[/timer].
For sheet cake
36 Follow instructions for mixing the batter.
37 Grease and flour a 13x9-inch baking dish.
38 Pour the batter in the baking dish and smooth top.
39 Bake for 30 to 35 minutes at 350°. Start checking for doneness at 25 minutes just to be on the safe side.
40 Cool completely in pan and frost with your favorite frosting.