Yellow Butter Cake with Chocolate Frosting

From Martha Stewart

Makes one 9-inch layer cake

Ingredients
    24 ounces best-quality semisweet chocolate, finely chopped
    1/2 cup plus 1 tablespoon Dutch-process cocoa powder
    1/2 cup plus 1 tablespoon boiling water
    3 sticks (1 1/2 cups) unsalted butter, room temperature
    1/2 cup confectioners' sugar
    Pinch of salt
    Yellow Butter Cake
    Colored sprinkles (optional)

Directions
Place chocolate in a heatproof bowl set over (but not touching) simmering water. Turn off heat; stir occasionally until chocolate has melted completely, about 15 minutes. Set bowl on countertop, and let chocolate cool to room temperature, 25 to 30 minutes. Meanwhile, combine cocoa powder and boiling water in a small bowl; stir until cocoa is dissolved.
   
In the bowl of an electric mixer fitted with the paddle attachment, beat butter, confectioners' sugar, and salt on medium-high speed until light and fluffy, 3 to 4 minutes. Add melted chocolate; beat on low speed until combined, 1 to 2 minutes, scraping down the sides of the bowl as needed. Beat in cocoa mixture.
   
Using a serrated knife, trim tops of cakes to make level; brush off crumbs. Place four strips of parchment paper around perimeter of a serving plate or lazy Susan. Place the first layer on the cake plate, and spread top with 3/4 cup of frosting. Place the second layer on top, bottom side up, and spread top with 3/4 cup of frosting; repeat process with the third layer. Place the remaining layer on top of the third layer, bottom-side up; insert a dowel into the center of cakes if necessary. Spread entire cake with remaining frosting. Decorate with sprinkles, if desired.