
Fill a small pot with 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.