In a small saucepan, melt butter over medium heat. Stir in brown sugar and flour. Stir in water. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 1 minute more. If not using sauce immediately, cover with plastic wrap. If sugar crystals begin to form, reheat in microwave, uncovered, on 100 percent power (High) for 10 to 20 seconds and stir. Makes about 1-1/2 cups.