In 2-quart saucepan, melt butter over medium heat. Stir in brown sugar. Heat to boiling, stirring constantly. Reduce heat to low. Boil and stir 2 minutes. Stir in milk. Heat to boiling; remove from heat. Cool about 45 minutes or until lukewarm. Gradually stir in powdered sugar. If frosting is too thin, place saucepan of frosting in bowl of cold water. Beat frosting until smooth and thick enough to spread. If frosting becomes too thick, stir in additional milk, 1 teaspoon at a time.