Melt the butter in a medium heavy-bottomed saucepan (not non-stick) over medium heat.
Add the brown sugar, salt and heavy cream and stir the mixture with a rubber spatula until it's well blended.
Bring the mixture to a boil, scraping down the sides occasionally, and boil it for 4 to 5 minutes.
Remove the mixture from the heat and stir in the vanilla extract.
Transfer the butterscotch sauce to a container to cool.
