In a small heavy saucepan combine sugar and water and bring to a boil, stirring until sugar is dissolved. Boil syrup, without stirring but swirling pan occasionally, until deep golden caramel. Remove pan from heat and add cream slowly (mixture will bubble up). Cook sauce over low heat, stirring, until smooth and caramel is dissolved. Add butter and stir until melted. Serves 6 to 8.