2 cups heavy cream
1/2 cup granulated sugar
8 ounces semi-sweet chocolate, coarsely chopped
1/4 cup unsweetened natural or dutch-process cocoa powder
2 teaspoons pure vanilla extract
toppings: marshmallows, chocolate candies, candy canes, chocolate syrup, salted caramel, etc
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 6 hours or on high for 4 hours. Make sure you whisk it every 45 minutes - 1 hour to make sure the chocolate isn't burning on the bottom. Serve warm with marshmallows or other optional toppings. Store leftovers in the refrigerator and warm up on the stove or in the microwave.