Allow butter and eggs to stand at room temperature for 30 minutes. Meanwhile, lightly grease bottoms of two 8x8x2-inch square or 9x1-1/2-inch round cake pans. Line bottom of pans with waxed paper. Grease and lightly flour waxed paper and sides of pans. Or grease one 13x9x2-inch baking pan. Set pan(s) aside.

In a mixing bowl, stir together the flour, cocoa powder, baking soda, baking powder; and salt; set aside.

In a large mixing bowl, beat butter with an electric mixer on medium to high speed for 30 seconds. Gradually add sugar, about 1/4 cup at a time, beating on medium speed until well combined (3 to 4 minutes). Scrape sides of bowl; continue beating on medium speed for 2 minutes. Add eggs, one at a time, beating after each addition (about 1 minute total). Beat in vanilla.

Alternately add flour mixture and milk to beaten mixture, beating on low speed just until combined after each addition. Beat on medium to high speed for 20 seconds more. Spread batter evenly into the prepared pan(s).

Bake in a 350 degree F oven for 35 to 40 minutes for 8-inch pans and the 13x9x2-inch pan, 30 to 35 minutes for 9-inch pans, or until a wooden toothpick inserted in the center comes out clean. Cool cake layers in pans for 10 minutes. Remove from pans. Peel off waxed paper. Cool thoroughly on wire racks. Or place 13x9x2-inch cake in pan on a wire rack; cool thoroughly. Frost with Chocolate-Sour Cream Frosting. Makes 12 to 16 servings.

In a large saucepan, melt 1, 12-ounce package (2 cups) semisweet chocolate pieces and 1/2 cup butter over low heat, stirring frequently. Cool for 5 minutes. Stir in 1, 8-ounce carton dairy sour cream. Mixture may be slightly warm when powdered sugar is added. Gradually add 4-1/2 to 5 cups powdered sugar (about 1 pound), beating with an electric mixer until smooth. This frosts tops and sides of two 8- or 9-inch cake layers. (Halve the recipe to frost the top of a 13x9x2-inch cake.) Cover and store frosted cake in the refrigerator.