Beat together the butter and sugar for the cake in a mixing bowl.
Add in the flour, cocoa powder, eggs and vanilla extract until well combined.
Fold in the chopped pecans.
Spread the batter in an even layer in a well greased 9×13 baking dish.
Bake for 25 minutes until cooked completely through.
Immediately pour the marshmallows on top of the cake when removed from the oven and let them sit. Once they are soft and melted, spread them evenly over the top of the cake.
Mix together the ingredients for the icing until smooth and creamy.
Spread the icing over the top of the cooled cake, and serve.