In a microwave-safe bowl or cup, microwave one cup of milk to scalding (not boiling). This can also be done in a small saucepan on the stove top.
Gently whisk in the 1/4 cup cocoa powder into the milk.
In a 4-quart (or larger) slow cooker, combine sweetened condensed milk, whole milk, heavy whipping cream, vanilla extract and semi-sweet chocolate chips.
Finally stir in the cocoa and milk mixture that you made first.
Cover and cook on low for about 2 hours and stirring often as it begins to warm up. Stirring is very important because you want to make sure those chocolate chips are melting evenly and getting incorporated into the rest of the mixture. You don't want a big clump of melted chocolate on the bottom.
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!
Video
Notes
Edit: I updated this recipe from the first time I posted it. I originally mixed the cocoa powder straight in with the all the other ingredients but updated it to mix with warm milk first to make sure it is fully combined before adding to the rest of the ingredients. in the slow cooker.
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!