Place all of the ingredients into a large slow cooker. (Here’s one I own and love!) Vigorously whisk to get the cocoa powder as mixed in as possible—you don’t want it all floating on the top. Keep whisking for a minute or 2 until it’s all combined.

Cook on low for 5 hours or on high for 3 hours, whisking the mixture every 45 minutes to 1 hour to ensure the chocolate isn’t burning on the bottom.

Ladle the hot chocolate into mugs and top with marshmallows or other optional toppings.

Store leftovers in the refrigerator and warm up on the stove or in the microwave.