Preheat oven to 350°.
Cream margarine, and gradually add sugar, beating well at medium speed of a mixer. Add eggs and egg whites, 1 at a time, beating well after each addition.
Combine buttermilk and baking soda; stir well. Set aside. Combine flour, cocoa, baking powder, and salt; stir well. Add dry ingredients to creamed mixture alternately with buttermilk mixture, beginning and ending with dry ingredients. Stir in vanilla.
Pour batter into a 12-cup Bundt pan coated with baking spray. Bake at 350° for 45 minutes or until a wooden pick inserted in center comes out clean. Let cool in pan 10 minutes on a wire rack; remove from pan. Let cool completely on wire rack. Sift powdered sugar over top of cake.