Melt butter in a pan,
Add cocoa powder and milk. Stir!!!!
When is just starts to boil, remove from the heat.
Whisk in the powdered sugar-sift it as you add it.
Pour over brownies and cool to room temp.
Pop in fridge to set up.
Cut into squares. If the top cracks when you cut, let them sit a few mins at room temp before cutting.