Heat oven to 350°F. Grease and flour 13x9x2-inch baking pan.
In large bowl, beat butter, shortening, sugar and vanilla until light and fluffy. Add eggs; beat well. Stir together flour, cocoa, baking powder, baking soda and salt; add alternately with milk to butter mixture, blending well. Pour batter into prepared pan.
Bake 35 to 40 minutes or until wooden pick inserted in center comes out clean. Cool in pan on wire rack. Frost as desired. 12 to 15 servings.