Line three 9-in. round baking pans with waxed paper; grease and flour the pans and paper. Set aside.
In a large bowl, cream butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Beat in vanilla. Combine the flour, cocoa, baking soda, salt and baking powder; add to the creamed mixture alternately with milk, beating well after each addition.
Transfer to prepared pans. Bake at 350° for 20-25 minutes or until a toothpick inserted near the center comes out clean. Cool for 10 minutes before removing from pans to wire racks to cool completely.
For frosting, in a small heavy saucepan, melt chips and butter over low heat. Transfer to a large bowl; cool for 5 minutes. Stir in sour cream. Gradually beat in confectioners' sugar until smooth. Spread between layers and over top and sides of cake. Garnish with chocolate curls if desired. Refrigerate leftovers. Yield: 16 servings.