In a large bowl whisk the dry ingredients together and add the wet ingredients; beat for 5 minutes.
Pour into a well greased Pyrex 13 x 9-inch baking pan and bake at 350° F for 20-25 minutes or until done.
To make icing: place cocoa, sugar, margarine and milk into a medium saucepan. Boil 2 minutes then remove from heat and add vanilla. Cool to desired consistency (about 5-10 minutes) then spread onto cake quickly before it gets too hard. Icing should be smooth and firm when done.