Combine the sugar and syrup in a heavy 3-quart saucepan over medium-high heat. Stir in the water and bring to a slight simmer, then reduce the heat and cook—without stirring—for about 20-30 minutes, or until the syrup reaches a rich amber color. (If you don’t get a rich, dark color at this stage, the finished sauce will be quite pale.)
Whisk in the cream carefully. (The caramel will froth and sizzle, which is why the seemingly large pan is used for the small amount of liquid.) If the caramel seizes into a giant blob, don’t panic. Just continue to whisk and it will melt.
Remove from the heat and vigorously whisk in the butter. Serve hot, or refrigerate if not serving right away. Warm the caramel before serving.