In a medium saucepan, add the brown sugar, cream, butter, and salt. Cook over medium-low heat until the butter is melted, stirring occasionally. Increase the heat to medium, bring the mixture to a simmer, and cook for 3-4 minutes, stirring constantly.
Off the heat, stir in the vanilla. The sauce will be thin but will thicken as it cools.
Pour the sauce into a jar or bowl to cool. Refrigerate until ready to serve.
Warm lightly (don't bring to simmer or boil or it can separate or turn grainy) before serving or serve straight from the refrigerator. Serve over ice cream, pie, blondies or whatever treat calls for butterscotch sauce!
Brown Sugar: the sauce will be quite a bit darker in color if using dark brown sugar. Heavy Cream: I use Darigold 40% heavy cream.