1 cup butter, softened
3 cups packed brown sugar
4 large eggs, room temperature
2 teaspoons vanilla extract
2-2/3 cups all-purpose flour
3/4 cup baking cocoa
3 teaspoons baking soda
1/2 teaspoon salt
1-1/3 cups sour cream
1-1/3 cups boiling water
Frosting:
1/2 cup butter, cubed
3 ounces unsweetened chocolate, chopped
3 ounces semisweet chocolate, chopped
5 cups confectioners’ sugar
1 cup sour cream
2 teaspoons vanilla extract
Preheat oven to 350°. Grease and flour three 9-in. round baking pans.
In a large bowl, cream butter and brown sugar until light and fluffy, 5-7 minutes. Add eggs, 1 at a time, beating well after each addition. Beat in vanilla. In another bowl, whisk flour, cocoa, baking soda and salt; add to creamed mixture alternately with sour cream, beating well after each addition. Stir in water until blended.
Transfer batter to prepared pans. Bake until a toothpick comes out clean, 30-35 minutes. Cool in pans 10 minutes; remove to wire racks to cool completely.
For frosting, in a metal bowl over simmering water, melt butter and chocolates; stir until smooth. Cool slightly.
In a large bowl, combine confectioners’ sugar, sour cream and vanilla. Add chocolate mixture; beat until smooth. Spread frosting between layers and over top and side of cake. Refrigerate leftovers.