1. In a heavy bottomed saucepan over low heat, bring the cream just to a boil then add the brown sugar and stir just until it dissolves. Add small pieces of butter until completely melted, then stir in the chocolate chips.
2. Wait 3 to 5 minutes so that the chocolate is very sold before folding it in with a spatula. Stir in the salt and vanilla and blend until well incorporated. Let cool for 15 minutes; it will thicken as it cools.
(Can be stored up to 1 week in the refrigerator in a glass jar. Heat until warm but not hot before serving.