In 3 1/2- to 4-quart slow cooker, mix unsweetened chocolate, chocolate chips and condensed milk. Cover; cook on High heat setting 30 minutes, stirring every 10 minutes or until chocolate is melted.
Stir in 2 cups of the milk, using wire whisk, until smooth. Gradually stir in remaining 6 cups milk and the vanilla.
Cover; cook on High heat 2 hours longer or until hot.
Just before serving, stir mixture again with wire whisk. Reduce heat setting to Low. To serve, have guests ladle hot chocolate into cups and add desired Toppings.