In a medium saucepan, combine the brown sugar, corn syrup and heavy cream and bring to a boil over moderate heat. Cook, stirring constantly, until slightly darkened and thickened, about 4 minutes. Add the water, butter and vanilla and stir until smooth. Serve warm over the vanilla ice cream.
The caramel sauce can be refrigerated for up to 1 week. Reheat on the stove before serving.