In a microwave, melt chocolate; stir until smooth. Set aside. In a large bowl, cream butter and brown sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Beat in vanilla and melted chocolate.
Combine the flour, baking soda, salt and baking powder; add to the creamed mixture alternately with water and sour cream. Transfer to two greased and floured 9-in. round baking pans.
Bake at 350° for 25-30 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 butter and chocolate over low heat. Remove from the heat; cool for 5 minutes. In a large bowl, beat the confectioners' sugar, milk and vanilla until smooth. Gradually beat in chocolate mixture until frosting is light and fluffy. Spread between layers and over top and sides of cake. Refrigerate leftovers. Yield: 12 servings.