In a 4-quart (or larger) slow cooker, combine all the ingredients. Give it a good stir. Updated to add: Some folks felt it was easier to heat up a cup of milk, then stir in the cocoa. It was mentioned that the cocoa blends in better this way. Then the warm milk mixture is then stirred in with the rest of the ingredients.
Cover and cook on low for about 2 hours. Stir occasionally as it warms up. It's ready when all the chocolate chips have melted and mixture is warm.
Ladle into mugs and top with marshmallows or whipped cream and a mini candy cane!
For peppermint hot chocolate, add in a couple of teaspoons of peppermint extract instead of vanilla extract. For an adult beverage, try adding some Kahlua, Peppermint Schnapps or Rum Chata!