01 -
In a microwave-safe bowl, heat 1 cup of heavy cream for 1 minute on high. Whisk in the unsweetened cocoa powder until it's completely dissolved and smooth.
02 -
In a 4-quart crockpot, add the cocoa mixture, the remaining 2 1/2 cups of heavy cream, whole milk, sweetened condensed milk, and vanilla extract. Whisk everything together really well until it's all combined.
03 -
Stir in the semi-sweet chocolate chips so they're evenly distributed throughout the mixture.
04 -
Cover the crockpot with the lid and set it to LOW. Heat for 2 to 3 hours until the chocolate chips have completely melted and everything is warm and smooth. Whisk every 20 to 30 minutes to prevent anything from sticking to the bottom and to make sure the chocolate melts evenly.
05 -
Once everything is melted and smooth, give it a good whisk and switch the crockpot to WARM. You can keep it warm for up to 2 hours. Ladle into mugs and top with mini marshmallows if you like!