Preheat the oven to 350 degrees. Place in a saucepan butter, coke, semi-sweet baking square and bring to a boil remove from heat and add marshmallows and stir until all is blended and the marshmallows are disappear. Set aside to cool enough to tepid or room temperature.
In mixing bowl blend the shortening, oil, vanilla, and sugar until fluffy. Add eggs and buttermilk and beat until smooth. In another bowl mix flour with baking soda, cocoa, baking powder and salt, add half of the flour mixture to shortening, egg mixture, and beat. Scrape bowl time to time to clean the sides off so ingredients are blended in the batter. Add cooled coca-cola mixture and beat. Add remainder of flour and beat until completely smooth about 3 - 4 minutes. The batter will be thick. Place batter into a 13" x 9" x " 2 greased and floured baking pan. Set pan on top shelf in a preheated 350 degrees oven for 35 - 40 minutes. Check for completed baking time with a toothpick. If the toothpick comes out clean by sticking in the middle of the cake its time to come out of the oven and cool.
Frosting In a bowl cream together margarine, cocoa, confectioners' sugar. Slowly add the 6 tablespoons of cream or milk. Add in vanilla. Frost warm cake. Serve with natural vanilla bean ice cream and enjoy.