In 2-quart heavy saucepan, heat 1/2 cup butter, 2 cups brown sugar and the whipping cream to boiling, stirring frequently. Boil and stir 1 minute; remove from heat. Stir in powdered sugar and vanilla until smooth. Stir in 2 cups pecans. Cool 10 to 20 minutes, stirring occasionally, until frosting begins to thicken. Spread warm frosting over cake. Cool before cutting.