In a large bowl, sift together the flour and sugar.
Mix together the margarine, crisco, cocoa, & coke. Boil for 1 minute in a saucepan. Pour this mixture over the flour sugar mixture. Add the buttermilk, baking soda and eggs. Mix well. Fold in the marshmallows. Bake in a 9x13 greased pan at 350* for 45-50 minutes.
Make the icing by placing the margarine, cocoa and coke in a saucepan and boiling for 1 minute. Remove from heat and add the powdered sugar, vanilla and nuts mixing well. Evenly pour over warm cake.