In a crockpot, add the whole milk, heavy cream, sweetened condensed milk, chocolate chips and cocoa powder. Stir everything together to combine.
Set the crockpot to low and cook for 2 hours, stirring occasionally to ensure the chocolate melts evenly.
After 2 hours, add the vanilla extract and give it one final stir.
Ladle into mugs and top with whipped cream, marshmallows or a sprinkle of crushed candy canes for extra holiday cheer.
Add a Peppermint Twist: A few drops of peppermint extract can give a minty holiday flavor!
Serving Suggestion: For an adult version, add a splash of Irish cream or peppermint schnapps to each mug.