Bring sugar, cream, and butter to boil in heavy medium saucepan over medium-high heat, whisking constantly until sugar dissolves. Boil until caramel thickens enough to coat spoon thickly, whisking often, about 10 minutes. DO AHEAD Caramel sauce can be made 5 days ahead. Cover; chill. Whisk over low heat until warm before using.
Makes 1.5 cup of sauce.