Turn this kid's cold-weather comfort drink into an adult beverage with the addition of your favorite liqueur or just a shot of brandy.
In a pot over medium-high heat, bring the milk to a simmer. Add the Nutella, stirring until it has melted and become incorporated. Add the cinnamon stick and bring to a simmer.
Ladle into mugs and serve with fresh oven toasted marshmallows. (If serving with marshmallows, toast on a foil-lined baking sheet under the broiler for 30 to 45 seconds, until golden brown.)