Preheat oven to 350°. Line three 9-in. round baking pans with waxed
paper; grease and flour the pans and paper.
In a large bowl, cream butter and sugar until light and fluffy, 5-7
minutes. Add eggs, 1 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 side of cake. If desired, top with
chocolate curls. Refrigerate leftovers.