Chocolate-and-Citrus Cassata
Directions
Make the cake layers
Preheat oven to 350°F. Lightly grease 3 (8-inch) round cake pans with cooking spray; line bottoms with parchment paper, and set aside. Sift together flour, cocoa, baking soda, and baking powder. Stir in salt.
Place brown sugar, butter, and mayonnaise in bowl of a heavy-duty stand mixer fitted with the paddle attachment; beat on medium speed until light and fluffy, about 3 minutes. Add eggs, 1 at a time, beating until fully incorporated after each addition. Stir in vanilla. With mixer running on low speed, add flour mixture alternately with buttermilk in 3 additions, beginning and ending with flour mixture. Beat until just incorporated after each addition. Divide batter evenly among prepared pans.
Bake in preheated oven until a wooden pick inserted in center comes out clean, 16 to 20 minutes. Let cakes cool in pans on wire racks 10 minutes.
Remove cakes from pans, and let cool completely on racks, about 1 hour.
While cakes cool, make the buttermilk-chocolate frosting
Bring 1 inch of water to a simmer in a medium saucepan over medium. Combine unsweetened chocolate, bittersweet chocolate, and butter in a medium-size heatproof bowl. Place bowl over simmering water, ensuring base of bowl does not touch simmering water. Cook, stirring often, until mixture is melted and smooth, 8 to 10 minutes. Remove from heat, and let cool slightly, 10 to 15 minutes.
Combine powdered sugar, buttermilk, and vanilla in bowl of a heavy-duty stand mixer fitted with the whisk attachment; beat on low speed until
smooth, about 1 minute. With mixer running on low speed, gradually add cooled chocolate mixture, beating until just incorporated, about 30 seconds. Set frosting aside, uncovered, and let stand, stirring occasionally, until completely cool, 1 to 2 hours. (Frosting should thicken as it cools. Do not chill.)
