To make sauce: In 1 1/2-quart saucepan, melt 4 tablespoons butter over medium heat. Add brown sugar; cook 3 to 5 minutes, stirring frequently, until sugar dissolves. Reduce heat to low; carefully stir in whipping cream and 1 teaspoon vanilla. Return heat to medium; heat to simmering. Cook 1 to 3 minutes, stirring constantly, until mixture is slightly thickened. Cool 45 minutes before serving. Sauce should still be pourable. Using sharp knife, cut pie, and serve drizzled with sauce.