Melt the butter in a small saucepan over medium heat.
Stir in cocoa, chocolate, and salt.
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.