This is from the back of a Hershey's cocoa box.
Hershey's "Perfectly Chocolate" Chocolate Frosting
1/2 cup (1 stick) butter
2/3 cup Hershey's cocoa
3 cups powedered sugar
1/3 cup milk
1 teaspoon vanilla
Melt butter. Stir in cocoa. Alternately add powder sugar and milk, beating to spreading consistency. Add small amount of additional milk, if needed. Stir in vanilla. Makes about 2 cups of frosting.