Grease and flour a 9 x 13-inch pan and set aside
In a large bowl combine flour and sugar. In a saucepan combine the cocoa, Coca-Cola, butter and marshmallows; bring to a boil. Combine the boiled mixture with the flour and sugar mixture.
In a separate bowl mix eggs, buttermilk, baking soda and vanilla; add to the first mixture.
Pour into prepared pan and bake at 350 degrees for about 35 minutes, until cake tests done.
Frosting:
In a saucepan, bring butter, cocoa and Coca-Cola to a boil, Stir in the sugar and mix well, Stir in nuts. Spread over the cake while both cake and frosting are still warm.