Set a medium saucepan over medium heat, then add the butter and melt it, stirring constantly. Whisk in the cream, brown sugar, and 1/2 tsp (2 ml) of the salt. Bring to a gentle boil, lower the heat and simmer for 5 minutes, whisking a few times throughout. Remove the butterscotch sauce from the heat, then whisk in the vanilla. Taste the sauce (careful not to burn yourself!). Adjust the salt level to your liking.
SERVING: You can serve butterscotch sauce cold from the fridge, room temperature, or warm. To reheat, simply microwave the butterscotch sauce for a few seconds at a low heat setting, stirring every 10 seconds until the sauce is loose and warm.
STORAGE: Butterscotch sauce thickens and becomes paler in color as it cools. Even refrigerated, it remains soft enough to use straight out of the fridge–very handy for sudden cravings! Keep the sauce refrigerated for up to 2 weeks. Freezing is not advised.