test done in the center. While brownies bake, prepare the frosting. In a saucepan, combine the butter, cocoa and milk. Heat to boiling, stirring. Mix in the powdered sugar and vanilla until frosting is smooth. Pour warm frosting over brownies as soon as you take them out of the oven. Cool. Cut into 48 bars. * 2 tsp vinegar or lemon juice mixed into 1/2 cup milk may be substituted for the buttermilk.