Sift together sugar and flour in a large bowl.
Combine margarine, shortening, cocoa and water in a saucepan.
Bring to a rapid boil, then pour over flour sugar mixture; stir well.
Add buttermilk, eggs, baking soda and vanilla; mix well.
Pour batter into a greased 15 1/2x10 1/2 inch baking pan (jelly roll pan).
Bake in a 400°F oven 20 minutes.
Five minutes before cake is done, prepare Icing.
Spread icing on cake while it is still hot and in the pan.