Marshmallows may be chopped or miniature. Sift together flour and sugar, stir in marshmallows and set aside. Bring to a boil, shortening, margarine, cocoa and Coke. Remove from heat and pour over dry ingredients. Stir in buttermilk, baking soda and eggs. Do not beat this cake mixture. Pour into a greased 13x9x2-inch pan and bake in a preheated 350 deg. oven for 45 minutes or until done. Cool 30 minutes before frosting cake. Coke Cake Frosting: Bring margarine, cocoa and Coke to a boil. Boil a minute, remove from heat and gradually add sugar. Stir in pecans.