01 -
In a microwave-safe bowl, combine unsweetened cocoa powder with 1 cup of heavy cream. Microwave briefly, then whisk until the mixture is completely smooth.
02 -
Pour the cocoa mixture into the insert of a 4-quart crockpot. Add the remaining 2 1/2 cups heavy cream, whole milk, sweetened condensed milk, and vanilla extract. Whisk thoroughly to blend until homogeneous.
03 -
Incorporate the semi-sweet chocolate chips into the liquid mixture, stirring gently to distribute.
04 -
Secure the crockpot lid and cook on LOW for 2 to 3 hours, stirring every 20 to 30 minutes to encourage even melting and prevent scorching, until the chocolate chips are fully dissolved and the mixture is velvety.
05 -
Switch the crockpot setting to WARM. Ladle the hot chocolate into mugs and garnish each with mini marshmallows as desired.