Combine flour, sugar in large mixing bowl. Heat butter, cocoa and Coke to boiling. Pour over flour mixture; mix well. Add buttermilk, eggs, soda, vanilla and mix. Add marshmallows and mix well. Batter will be thin with marshmallows on top.
Bake in flat sheet cake pan at 325 degrees for 40-50 minutes or until center of cake is done. Do not remove from cake pan. Top with Coca Cola Icing while cake is still hot.
Coca Cola Icing:
1/2 c. butter
3 T. cocoa
6 T. Coca Cola
1 c. chopped toasted pecans
1/2-1 box powdered sugar (depending on desired stiffness)
Combine butter, cocoa, and Coke and heat to boiling. Pour over sugar; beat well. Add pecans. Spread over hot cake. Leave cake in pan.