Heat margarine, coca and coke until ready to boil; remove from heat. Add flour, sugar and baking soda, mix well. Mix in a seperate bowl the eggs, vanilla, buttermilk and marshmellows until smooth then add to other mix and mix well. Pour into a greased 13x9 cake pan and bake at 350 for 30 minutes.
For the icing heat margarine, cocoa, and coke until melted. Remove from heat and add sugar. Ice cake as soon as cake comes out of the oven.