1/2 c. butter
1 c. firmly packed brown sugar
1/4 c. whipping cream
2 1/2 c. sifted powdered sugar
1 tsp. vanilla extract

Melt butter in a heavy saucepan. Add brown sugar; cook over low heat, stirring constantly, until sugar dissolves. Do not boil. Remove from heat. Stir in whipping cream. Add powdered sugar and vanilla; beat at high speed of an electric mixer until smooth.