Combine flour and sugar in a bowl. In a saucepan, heat butter, cocoa and coke until it boils. Pour over flour and sugar mixture. Mix well.
Add eggs, buttermilk, soda, vanilla, and marshmallows. Mix well. Pour into greased and floured 9x13 pan. Bake 30-40 minutes at 350. Ice while hot.
Icing- Combine butter, cocoa ad coke. Bring to a boil. Pour over powdered sugar and beat well. Add nut and spread over hot cake.