Place in sauce pan, 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 have disappeared. Set aside and let 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. Mix flour with baking soda, cocoa, baking powder and salt, add half the flour mixture to shortening, egg mixture, and beat. Scrape bowl time to time to clean off so ingredients are blended in batter. Add cooled coca cola mixture and beat. Add remainder of flour and beat until completely smooth about 3-4 minutes. Batter will be thick. Place batter in a 13x9x2 greased pan.
Set pan on top shelf in a preheated 350F degree oven. Bake for 35-40 minutes or until wooden pick comes out clean.
FROSTING:Cream butter and cocoa; add vanilla, hershey syrup and coca cola until smooth. Add powdered sugar a little at a time and beat constantly. Scrape the sides of the bowl time to time to get all ingredients into the frosting. If the frosting seams a little dry add a little more coca cola. Frost warm cake and serve with vanilla ice cream.