Food Cake with Milk Chocolate Sour Cream Fudge
For the Cake
3/4 cup natural unsweetened cocoa powder
3 ounces high-quality milk chocolate (such as Lindt, Perugina, or Valrhona), chopped
3/4 cup boiling water
3/4 cup buttermilk
2 cups cake flour (*see substitute below)
1 1/2 teaspoons baking soda
3/4 teaspoon salt
1 cup (packed) dark brown sugar
3/4 cup sugar
3/4 cup canola oil
3 large eggs
1 1/2 teaspoons vanilla extract
For the Frosting
24 ounces high-quality milk chocolate (such as Lindt, Perugina, or Valrhona), chopped
3/4 cup (1 1/2 sticks) unsalted butter, room temperature
2 cups sour cream
1/4 cup light corn syrup
Position rack in center of oven; preheat to 350°F. Butter two 9-inch cake pans with 2-inch-high sides. Line bottom of pans with parchment paper. Combine cocoa powder and milk chocolate in medium bowl. Pour boiling water over; whisk until mixture is smooth. Whisk in buttermilk. In another medium bowl, whisk flour, baking soda, and salt .
Using electric mixer, beat both sugars, oil, egg, and vanilla in large bowl until well blended. Add flour and cocoa mixtures; beat until blended (batter will be thin). Divide batter between pans.
Bake cakes until tester inserted into center comes out with some crumbs attached, 30 to 34 minutes. Cool in pans on racks 15 minutes. Turn out onto racks; peel off parchment. Turn over; cool on rack for at least an hour or until completely cool.
For the Frosting
Place chocolate in large metal bowl. Set bowl over saucepan of simmering water and stir until melted and smooth. Remove bowl from over water. Add butter and stir until melted, then add sour cream and corn syrup and whisk until smooth. Let frosting stand at room temperature until thick enough to spread, about 20 minutes.
When cakes are cool, using serrated knife, trim top of cakes to make level. Cut each cake horizontally in half. Place 1 cake layer, cut side up, on platter. Spread about 2/3 cup frosting (I eye-balled it) over, leaving 1/2-inch border. Top with second cake layer, cut side down. Spread 2/3 cup frosting over, leaving 1/2-inch border. Top with third cake layer, cut side up. Spread 2/3 cup frosting over, leaving 1/2-inch border. Top with remaining cake layer, cut side down. Spread 3/4 cup frosting over top and sides. This is a thin layer – cake will show through. Chill in the refrigerator until frosting is set, about 30 minutes. Keep remaining frosting at room temperature. Spread remaining frosting evenly over top and sides of cake.
DO AHEAD Can be made 2 days ahead. Cover with cake dome and store at room temperature.
* Cake Flour Substitute: Fill 1 cup measure with 2 Tablespoons cornstarch, fill the rest with white flour. This is equivalent to 1 cup Cake Flour