Combine the chocolate, sugar, milk, and butter in a medium sauce pan. Cook over medium-heat until melted (about 6-7 minutes)

Bring to a light boil, stirring constantly. Once it has become a light boil, immediately remove from heat. Then stir in vanilla. Allow to cool for 5 to 10 minutes. It will thicken as it cools.

Store in the refrigerator for up to 1 month. Reheat in microwave in 30-second intervals until warm.