In a 2- to 3-quart pan over medium heat, tilt and swirl sugar often until melted and amber-colored, 5 to 7 minutes. Stir in cream; mixture will bubble and sugar will harden. Stir until sugar melts again and sauce boils, 6 to 8 minutes.
At once, pour sauce into a bowl. Stir in vanilla.
Let sauce cool for 10 minutes, then, if desired, stir in rum. Serve warm or cool.