Once the sugar has melted and is a nice caramel color, stir in the 3 tbsp. of butter. When it’s incorporated, slowly mix in the ⅓ cup of heavy cream. It will bubble up, but just keep stirring. Now that it looks like a sauce, add in the ½ tsp. of vanilla and ¾ tsp. of salt. Let it simmer on low heat for just a couple of minutes longer. Turn off the heat, and let it cool.