Into a medium bowl, pour in the melted butter and stir in the cocoa.
Alternately add 1 cup of powdered sugar and a little of the milk, beating with an electric mixer until you've added all of the powdered sugar and milk, and beat to spreading consistency. Add small amount of additional milk, if needed. Stir in the vanilla.
Notes
This frosting pipes well onto cupcakes or cake.
The recipe makes 2 cups of frosting, so you can frost a 9-inch layer cake, a 9x13-inch cake or 16 cupcakes.
This recipe can be easily doubled (or 1 1/2).
Hershey's chocolate frosting may be refrigerated in a sealed container for up to one week.