In a small saucepan melt butter; stir in brown sugar. Cook and stir until bubbly. Remove from heat. Add milk; beat vigorously with a wooden spoon until smooth. Add powdered sugar; beat by hand until mixture is easy to spread. Use frosting immediately. (If frosting thickens, stir in hot water, a few drops at a time, until of spreading consistency again.) Makes enough to frost the top of 13x9-inch cake (12 servings).