Melt butter in saucepan. Add brown sugar; cook over low heat 2 minutes, stirring constantly. Add milk, and cook and stir until mixture comes to a boil. Remove from heat, and cool about 10 minutes. Gradually add powdered sugar until frosting is of right consistency to spread, beating well after each addition. Makes enough to cover tops and sides of two 8- or 9-inch layers, or top and sides of a 9 X 9 X 2-inch cake.