Preheat oven to 350 degrees. Sift (or mix together well) flour, sugar, salt, cocoa and baking soda into a 9 x 13 ungreased baking pan. Make three long holes or canals; put oil in one, vanilla in the second and vinegar in the third. Pour the cold water over the mixture. Mix all of it together so that there are no lumps of dry ingredients (a wooden spoon works well.) Bake 30 - 40 minutes. While it cools, make frosting.
Put sugar, soy or rice milk, cocoa and margarine in a small saucepan and heat to a boil while stirring constantly. Boil 1 minute, add vanilla and remove from heat. It burns easily, so keep stirring the whole time! Place in refrigerator for 5 minutes, stir thoroughly and repeat until frosting is a thin consistency.
To assemble: pour frosting over cake and let sit 30 minutes to let the frosting set. If you can't wait, put it in the refrigerator for about 10 minutes.
Makes 12 servings.