Heat oven to 350 degrees. Grease and flour 13 x 9 x 2 inch baking pan. In large mixer bowl, beat butter, shortening, sugar and vanilla until light and fluffy. Add eggs; beat well. In separate bowl, 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 toothpick inserted in center comes out clean. Cool in pan on wire rack. Frost as desired.
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