Preheat oven 350 degrees.
Spray a 9 x 13 baking dish with a cooking spray
Blend the cake mix, cocoa powder, melted butter, Coca-Cola, buttermilk, eggs, and vanilla using an electric mixer.
Stop the machine and scrape down the sides and continue mixing two minutes.
Fold in the Marshmallows
Pour the batter into the 9 x 13 baking dish.
Smooth it out
Bake the cake 40 minutes about until it when tested the center comes out clean and bounces back when lightly pressed.
Melt the butter over medium heat in a saucepan.
Stir in the cocoa powder and Cola.
Let the mixture come to a boil stirring constantly.
Remove from the heat.
Stir in the confectioners' sugars until smooth and creamy.
Stir in the chopped pecans.