Add half of the heavy cream to the slow cooker. Whisk in the cocoa powder until combined. This step will require a few minutes of whisking to incorporate the powder thoroughly into the cream.
Add the remaining cream, milk, sweetened condensed milk, chopped chocolate, and vanilla. Stir well to combine.
Cook on low heat 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!
Don't add all ingredients at once. It's important to follow the directions in step 1 so that the cocoa powder mixes in easier. If you add everything into the slow cooker at once it will take a very long time to mix the cocoa powder in. It tends to want to just sit on top and not mix in, so I find adding ingredients incrementally in this way makes it easier.
Stir intermittently. Make sure you are whisking/stirring every 20 minutes or so. This will keep the chocolate from burning onto the sides of the crockpot.
Adjusting the thickness. This makes a very thick and creamy hot chocolate, which is thicker than a classic packet of hot chocolate from the grocery store. If you want it thinner, use 2 cups of heavy cream instead of 4, and add 2 cups of milk in its place. You can also use 2% milk to thin it out. I personally love it as is; it’s so luscious and decadent, a true treat!
Use good quality chocolate! You can use semi-sweet chocolate chips in place of the baking bar. I suggest using higher quality chocolate so it melts nicely.
How to Store Leftovers: Leftovers can be stored in an airtight container in the fridge and reheated on the stove or in the microwave.
How to make on the stove top: If you don't have a slow cooker, you can easily make this Polar Express Hot Chocolate on the stove. Follow the directions for adding the ingredients to a large soup pot or dutch oven and warm the hot chocolate over low heat until the chocolate bar pieces have melted. Keep on low heat uncovered and stir occasionally to keep the hot chocolate from burning.