Cake: In a sauce pan, bring Coca-Cola, oil, margarine, and cocoa to a boil. Mix the sugar, flour and salt, pour into the boiling liquid and beat well. Add the eggs, buttermilk, baking soda, and vanilla and beat well. Pour into a greased and floured sheet cake pan and bake at 350 degrees for 20-25 minutes.
Frosting: In a saucepan, combine the margarine, cocoa, and cream or milk and heat until the butter melts. Beat in the remaining ingredients. Spread on the hot cake. Cool and cut.