1/2 C. unsalted butter
3/4 C. unsweetened cocoa powder
2 ox. bittersweet chocolate, chopped
1/4 tsp. salt
1 (14 oz.) can sweetened condensed milk
1/2 C. heavy cream
1 Tbsp. pure vanilla extract

Melt the butter in a small saucepan over medium heat. Stir in the cocoa, chocolate, and salt, and cook until the chocolate melts. Slowly stir in the milk and cream and heat until the sauce is smooth and warm. Stir the sauce constantly and watch it carefully so it does not boil or stick to the bottom of the pan. Remove the sauce from the heat and whisk in the vanilla.
Store any leftover sauce in a covered jar in the refrigerator. To reheat add an extra tablespoon or two of cream and stir constantly over medium heat until hot.