In a dry heavy saucepan (about 2 quarts) cook sugar over moderate heat, stirring with a fork, until melted and cook without stirring, swirling pan, until a golden caramel. Remove pan from heat and carefully add water and brandy down side of pan (caramel will bubble and steam). Return pan to heat and simmer, stirring, until caramel is disolved. Stir in butter. Sauce may be made 3 days ahead and chilled, covered. Reheat sauce to warm before serving.