In a sauce Pan, Bring Coca-Cola, Oil, Margarine, and cocoa to a boil, mix the sugar, flour and salt, Pour in the boiling liquid and beat well. Add the eggs, Buttermilk, soda, and vanilla and beat well. Pour into a greased and floured sheet cake pan, and bake at 350 degrees for 20-25 minutes.
For Frosting: In a saucepan, combine the butter cocoa, and milk and heat until the butter melts, Beat in the remaining ingredients, and Spread on the hot cake. Cool and cut.