Add half of the heavy cream to the slow cooker. Whisk in the cocoa powder until combined, this might take a while.
Add the remaining cream, milk, sweetened condensed milk, chopped chocolate, and vanilla, stir to combine.
Place on low for 3 hours, stirring every 20 minutes or so until the chocolate is melted and fully combined. Keep on the warm setting for up to 3 hours for serving. If the hot chocolate gets too thick while it sits, you can stir in more milk to your desired thickness.
Serve with your desired toppings such as marshmallows, whipped cream, chocolate syrup, or crushed candy canes!
