Preheat oven to 350°. 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, 5-7 minutes. 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 until a toothpick inserted in the center comes out clean, 20-25 minutes. 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. If desired, top with chocolate curls. Refrigerate leftovers.