1 stick (4 ounces) unsalted butter, cut into tablespoons
1/4 cup rum
3 tablespoons sour cream
In a medium saucepan, bring the sugar and 1/2 cup of the water to a boil. Cook over moderate heat without stirring until a deep amber caramel forms, about 3 minutes. Gradually whisk in the remaining 1/2 cup of water; it may splatter, so be careful. Stir in the butter, rum and sour cream and serve warm.
The caramel sauce can be refrigerated for 2 days. Rewarm over moderate heat, stirring; do not boil.