4 oz. bittersweet chocolate, chopped
3 Tbsp. plus 1 and 1/4 cups butter softened, divided
2 cups sugar
5 large eggs
3 tsp. vanilla extract
2 and 1/4 cups all-purpose flour
2 tsp. baking powder
1/2 tsp. salt
1/2 cup sour cream
1/2 cup miniature semisweet chocolate chips, optional
3/4 cup butter, softened
6 and 3/4 cups confectioners’ sugar
2 tsp. vanilla extract
1/2 to 3/4 cup 2% milk
2 Tbsp. miniature semisweet chocolate chips
In top of a double boiler or a metal bowl over barely simmering water, melt chocolate and 3 Tbsp. butter; stir until smooth. Cool to room temperature.
Preheat oven to 375°. Line the bottoms of three greased 8-in. round baking pans with parchment paper; grease the paper.
In a large bowl, cream remaining butter and the sugar until light and fluffy. Add the eggs, one at a time, beating well after each addition. Beat in vanilla. Whisk the flour, baking powder and salt; add to creamed mixture alternately with the sour cream, beating well after each addition.
Remove 2 cups of the batter to a small bowl; stir in cooled chocolate mixture and, if desired, the chocolate chips, until blended. Drop both the plain and chocolate batters by tablespoonfuls into prepared pans, dividing batters evenly among pans. To make the batter level in pans, bang cake pans several times on counter.
Bake until a toothpick inserted in center comes out clean, 20-25 minutes. Cool in pans 10 minutes before removing to wire racks; remove paper. Cool completely.
In a large bowl, beat the butter until smooth. Gradually beat in confectioners’ sugar, vanilla and enough milk to reach desired consistency.
If cake layers have rounded tops, trim with a serrated knife to make level. In a microwave, melt the chocolate chips; stir until smooth. Cool slightly.
Place one cake layer on a serving plate; spread with 1/2 cup frosting. Repeat layers. Top with remaining cake layer. Frost top and sides of cake.
Drop chocolate by 1/2 teaspoonfuls over frosting. Using an offset spatula, smear chocolate to create marbling in the frosting.