Add all of the ingredients to a large slow cooker. Whisk the mixture vigorously for 1-2 minutes to incorporate the cocoa powder so that it does not sit on the top.
Cook on low for 2-3 hours, stirring every 45 minutes to one hour so that the chocolate does not burn to the bottom.
Ladle the hot chocolate into mugs and top with mini marshmallows for serving.
Notes
Leftover hot chocolate can be stored in a covered container in the refrigerator for 3-4 days. It can be reheated on the stovetop or in the microwave.