from williams-sonoma.com
2 1/3 cups all-purpose flour
1 cup unsweetened cocoa powder, sifted
1 1/2 tsp. baking powder
1/2 tsp. baking soda
1/2 tsp. salt
12 Tbs. (1 1/2 sticks) unsalted butter, at room temperature
2 cups firmly packed light brown sugar
2 tsp. vanilla extract
4 eggs, at room temperature
1 1/2 cups buttermilk, at room temperature
Preheat an oven to 350°F. Lightly butter the bottoms of two 9-by-2-inch round cake pans and line with parchment paper. Lightly butter the paper and the sides of the pans and dust with flour.
In a large bowl, sift together the flour, cocoa powder, baking powder, baking soda and salt; set aside.
In another large bowl, using an electric mixer, beat the butter on medium speed until smooth. Gradually add the brown sugar and continue beating until fluffy. Beat in the vanilla. Add the eggs one at a time, beating well after each addition.
Reduce the speed to low and add the flour mixture in three additions, alternating with the buttermilk and beginning and ending with the flour, beating after each addition.
Divide the batter between the prepared pans and spread it out evenly. Bake until a toothpick inserted into the center of a cake comes out clean, 25 to 30 minutes. Transfer the pans to a wire rack and let cool for 15 minutes. Let the cake layers cool completely before frosting.
Serve immediately or keep covered at room temperature until ready to serve. Makes one 9-inch cake; serves 10 to 12.
Frosting
4 Tbs. (1/2 stick) unsalted butter
1/4 cup heavy cream
10 oz. bittersweet chocolate, chopped
3/4 cup sour cream
1 cup confectioners’ sugar
In a medium-size, heavy saucepan over low heat, combine the butter and cream. Heat, stirring frequently, until the butter melts. Add the chocolate and whisk until melted and smooth, about 2 minutes. Remove from the heat; let cool to barely lukewarm, about 8 minutes.
Whisk in the sour cream until fully combined. Then whisk in the confectioners’ sugar. Let stand until thick enough to spread, about 10 minutes. If the frosting becomes too stiff to spread, rewarm briefly over low heat and whisk again until smooth.