In a mixing bowl (a quart-size Pyrex measuring cup also works well with many hand beaters, and the butter can be melted in it first) stir the cocoa powder into the melted butter. Stir in the vanilla, and then add half the sugar and half the milk. Stir to partially incorporate (I just use the turned-off beaters–this prevents a powdered sugar dust cloud), and then beat on medium speed, adding the remaining sugar and enough milk to achieve a smooth, creamy consistency. If necessary, add more milk by the scant-teaspoon. Frost the cooled cake and enjoy.