Makes about 2 cups [480 ml]
8 tablespoons [1 stick/112 g] unsalted butter
1 cup [200 g] packed dark brown sugar
1-1/2 cups [360 ml] heavy cream
2 teaspoons kosher salt
1 tablespoon vanilla extract
Place the butter, brown sugar, cream, and salt in a medium saucepan set over high heat. Once it comes to a boil, decrease the heat and simmer, stirring now and then, until slightly reduced, about 15 minutes. Turn off the heat and stir in the vanilla.
Serve warm. You can also cool it to room temperature, store it in a covered container in the refrigerator for up to a week, and then reheat in a saucepan set over low heat until warm.