Melt the butter in a heavy saucepan.
Add the milk and cocoa, bring to a boil while whisking.
The glaze will thicken when it comes to a boil.
Remove from heat, and beat in the sugar with an electric mixer until smooth and glossy.
Drizzle or dip your cakes, donuts or cookies, while the glaze is still warm. It thickens as it cools.
Reheat as necessary.