In a medium saucepan, combine the butter with all three sugars and bring to a boil, stirring constantly. Cook over moderately high heat for 2 minutes, stirring constantly. Add the cream and boil for 2 minutes longer. Transfer the caramel sauce to a pitcher. Stir in the vanilla and salt and refrigerate until cold, at least 2 hours.
The caramel can be refrigerated for up to 1 week. Whisk to combine before serving.