When bars are almost completely cooled, in 1 1/2-quart saucepan, melt butter over medium heat. Add brown sugar; cook 4 to 6 minutes, stirring frequently, until sugar dissolves. Reduce heat to low; carefully stir in whipping cream and vanilla. Return heat to medium, and heat to simmering. Cook 1 to 3 minutes, stirring constantly, until mixture is thickened. Cool 30 minutes before serving.