Preheat the oven to 425°. In a medium saucepan, combine the sugar and water. Boil over moderately high heat, without stirring, washing down any crystals that form on the side of the pan, until a deep amber caramel forms, 6 to 8 minutes. Remove from the heat and stir in the cream; the sauce will bubble up. Pour the caramel into a heatproof cup and let cool slightly. Stir in the balsamic vinegar; keep warm.