This is Nanny's butterscotch sauce recipe, taken straight off her handwritten card in my recipe box. It's rich, buttery, and ready in one pot in about ten minutes, no candy thermometer required.
In a large pot over medium heat, combine syrup, butter, salt, and brown sugar. Bring to a boil, and boil for 4 minutes. Adjust heat as needed so syrup does not boil over the pot.
Remove syrup from burner and let sit for 5 minutes.