Heat oven to 325 degrees. Grease and flour Bundt pan. Sift flour, salt and baking powder together; set aside. Cream margarine, shortening and sugar until fluffy. Add eggs, vanilla and cocoa; beat until smooth. Add sifted dry ingredients alternately with milk, beginning and ending with dry ingredients. Mix well. Remove 2 cups batter, and make cupcakes. (Bake the cupcakes while cake is baking, but bake them only 30-45 minutes.) Pour remainder of batter into prepared Bundt pan. Bake in preheated oven at least 1 hour, or until done. Let cake stand in pan for about 30 minutes; then invert onto wire rack. Powdered sugar may be sifted on top just before serving. (Sugar turns brown if put on in advance.) This cake is even better 1 or 2 days after it is made, as it becomes more moist. Freezes well.