Begin by adding 2 cups of the heavy cream to your slow cooker. Whisk in the cocoa powder gradually until it is fully incorporated. This step may take a few minutes to ensure there are no lumps.
Add the remaining 2 cups of heavy cream, whole milk, sweetened condensed milk, chopped chocolate, and vanilla extract to the slow cooker. Stir everything together until well combined.
Set your slow cooker on low and cook for 3 hours, stirring every 20 minutes to ensure the chocolate melts evenly and mixes well with the liquids.
Once the chocolate is fully melted and the mixture is smooth, switch the slow cooker to the “warm” setting to keep the hot chocolate ready to serve for up to 3 hours. If it thickens too much while sitting, you can stir in more milk to reach your desired consistency.
Serve the hot chocolate with your favorite toppings like marshmallows, whipped cream, chocolate syrup, or even crushed candy canes for a festive touch!