Melt butter in a sauce pan on medium heat.Add brown sugar and stir to mix it into the butter. Then add the cream and bring the sauce to a boil.Keep the sauce at a light boil for 5 minutes, reducing the heat of the burner if needed. By a light boil I mean the sauce should be bubbling but you don't want the bubbles to be stacking on top of each other, something that happens easily when cooking sugary sauces.Remove the pan from the heat. Add the salt and vanilla. Taste and add more of either if you want to pump up the flavor a bit more.Let the sauce cool to room temperature. Store in a sealed container in the refrigerator for up to 2 weeks. When chilled, the sauce is pretty solid so warm it back up in saucepan on medium heat for a few minutes if you need it to be pourable.