1. Combine first 3 ingredients in a small saucepan; stir with a whisk until blended. Add milk, stirring with a whisk until blended. Bring to a boil; reduce heat, and simmer, stirring constantly, 1 to 2 minutes or until sugar dissolves.
2. Remove from heat. Add chocolate and vanilla, stirring until sauce is smooth. Serve warm or at room temperature.