Sift the flour, white sugar, baking soda, salt and 6 tablespoons of the cocoa into a 9 x 13 inch baking pan.
Drizzle melted margarine and the vinegar over the flour mixture. Add the vanilla and cold water and stir well to remove lumps.
Bake at 350 degrees F (175 degrees C) for 35 minutes. Spread with chocolate frosting.
To Make Chocolate Frosting: Combine the confectioner's sugar, 6 tablespoons water, 3 tablespoons cocoa, and 1/2 cup of the margarine. Heat until melted and stir until smooth add vanilla and nuts if desired, and spread over cake.
