Cook brown sugar and butter in a heavy 2-qt. saucepan over medium heat, stirring constantly, 3 to 4 minutes or until butter is melted and mixture is smooth.
Gradually stir in whipping cream. Bring mixture to a boil over medium heat, stirring constantly; boil, stirring constantly, 3 minutes. Remove from heat; stir in vanilla. Cool 10 minutes before serving. Garnish, if desired.
Note: To make ahead, prepare recipe as directed. Cover and chill up to 1 week. Reheat in a microwave-safe bowl at MEDIUM-HIGH (70% power) 3 to 4 minutes or until bubbly, stirring at 1-minute intervals. Cool slightly.