In a small sauce pot over low heat, add 1 cup of milk and the Nutella. Stir constantly, until the Nutella has dissolved into the milk. You can do this in a microwave-safe dish, by heating up the Nutella and 1 cup of milk for 60 seconds. Remove it from the microwave and stir to dissolve.
Add the remaining milk to the rich chocolatey mixture, raise the heat to medium, and heat up until the hot chocolate is hot but not boiling.
Serve:
Pour into two mugs and top with marshmallows.
Notes
For one: 3 tablespoons nutella + 1 ⅓ cups milk. Dissolve the Nutella in ⅓ cup milk first, then add the rest of the milk. When making this for one, I've found the microwave method works best.