In a medium, heavy-bottomed saucepan, melt the butter over medium heat.
Use a rubber spatula to stir in the brown sugar, salt, and heavy cream.
Bring the mixture to a boil and cook for 5 minutes, while occasionally scraping down the sides.
Remove from heat and stir in the vanilla.
Transfer the butterscotch sauce to a jar and allow it to cool completely.
Notes
The sauce will thicken as it cools. Take care not to overcook it or it will thicken to a chewy candy texture.
Serve the sauce drizzled over ice cream, pies, and tarts.
Storage: Store in the refrigerator for up to 3 weeks. Rewarm gently before serving in a saucepan over low heat or in the microwave for 20-30 seconds.Makes about 1.5 cups.