Sift flour and sugar together. Place margarine, cocoa and cola beverage in saucepan. Bring to boil and add to flour mixture. Dissolve soda in buttermilk, combine remaining ingredients and mix well. Batter will be thin with marshmallows floating on top. Pour into well greased (or Pam spray) 13 x 10 x 1/2 inch pan. Bake at 350 degrees for 30-35 minutes.
Bring margarine, cocoa, and cola beverage to boil. Add remaining ingredients. Frost cake while warm.