Whisk sugar and cocoa together in the base of a slow cooker. This will help eliminate getting lumps in the cocoa.
Add whole milk, heavy cream, and milk chocolate. Whisk together.
Cook in slow cooker on low for 2 hours. Whisk occasionally to make sure the chocolate incorporates with the milk as it melts.
10 minutes before serving add two cups of marshmallows to the slow cooker. Put the lid on and let hot chocolate sit for 10 minutes. Gently stir the marshmallows and they'll make your hot chocolate extra decadent!
Serve with any, or all, of the desired toppings. Store leftovers in the refirgerator and microwave to reheat when you're ready to serve.
You may use 2% milk, but it won't be quite as creamy as if you use whole milk.