Preheat oven to 350° F. Grease a 9" x 13" pan and use 2 Tablespoons of cocoa to flour.
Sift together flour and sugar, stir in marshmallows and set aside.
In medium saucepan bring shortening, butter, cocoa and cola to a boil. Remove from heat and pour over dry ingredients. Stir in buttermilk, baking soda and eggs.
Do not beat this cake mixture! Mix well by hand. Pour into greased and floured (with cocoa) 13" x 9" x 2" baking pan and bake in preheated oven for 45 minutes or until it tests done when perforated in the middle with a toothpick. Cool 30 minutes before frosting cake.
Bring butter, cocoa and cola to a boil. Boil one minute; remove from heat and gradually add powdered sugar. Stir in pecans and frost cake.