Preheat oven to 350 degrees. Spray a 9 x 13" cake pan with Baker's Joy or grease and flour pan. In a large bowl, combine sugar, flour, baking soda and salt; set aside. In saucepan over medium heat, stirring constantly, bring shortening, butter, cocoa and coca cola to a boil. Pour cocoa mixture over dry ingredients and mix well. Add eggs, sour cream, marshmallow crème and vanilla; mix thoroughly. Pour batter into prepared pan. Bake for 35-45 minutes or until toothpick inserted in center comes out clean. Frost cake.
In a large saucepan, combine butter, cocoa cola, milk and cocoa. Bring to a boil, stirring constantly. Remove from heat and stir in reserved marshmallow crème. Gradually beat in powdered sugar. Add vanilla and nuts. Spread the icing over the hot cake. Cool completely.
Note: If using a 15x10x1" pan, bake at 350 degrees for 30-40 minutes or until toothpick inserted in center comes out clean. The batter will rise to the top of the pan while baking but will sink when removed from the oven. Use a large spoon to spread the icing over the hot cake. Cool completely.