Melt butter in a saucepan.
Add milk and cocoa.
Stir until it boils- about 2 minutes.
Remove heat and add vanilla and confectioner's sugar.
Keep warm over low heat.
Once you remove brownies from the oven, immediately spread the marshmallow creme on top. Be careful doing this and let it sit and melt a bit as you go.
Pour the warm icing over the creme and spread it.
Cool and cut!