Combine sugar, flour and cinnamon in a large bowl and mix well using a wire whisk. Combine butter, the 1 cup of coca cola and cocoa in a pot on the stove and bring to a boil stirring often. Remove from heat and pour over flour mixture. Mix with a spoon. Mix buttermilk and baking soda together and pour into cake mixture. Add marshmallows and continue stirring.(Your marshmallows will not melt) Add in eggs and vanilla extract. Pour into a sprayed 9 x 13 inch pan and bake in preheated 350 degree oven for 30 to 35 minutes, testing for doneness.
Bring margarine, cocoa and coca cola to a boil on top of the stove. Remove from heat and add vanilla and powdered sugar until frosting is of spreading consistency. You can either stir in nuts or sprinkle on top of the cake. Spread over hot cake.