Melt the butter in a small suacepan over low heat, about 2 minutes.

Stir in the cocoa powder and milk. Cook the glaze, stirring, until it thickens and comes to a boil, about 2 minutes.

Remove the pan from the heat and stir in 2 cups of confectioners' sugar, adding more if needed to make the mixture smooth and a little thicker than pancake syrup; don't let the glaze get too thick.

Ladle the warm glaze over a room temperature cake and let sit until the glaze hardens, about 30 minutes.