Fill a small pot with the water and bring to a boil. Whisk in the cocoa powder and sugar.
Reduce heat to medium and whisk in the milk. Cook over medium heat until the milk is heated but not boiling, about 3 minutes
Add chocolate chips and continue to whisk until the chocolate chips melt and the mixture becomes smooth, about 2 more minutes.
Remove from the heat; stir in vanilla extract.
Serve with mini marshmallows, if desired.
Notes
Storage: Leftovers can be cooled at kept covered in the ridge for 2 to 3 days. Reheat it on the stovetop to warm through to serve.Tips: You can make this hot chocolate with any kind of milk. Use a plant based milk like almond or oat for a vegan hot drink, or half and half for a creamier and more indulgent sip.