In a small pot, add milk and bring to a very hot temperature on medium heat (it doesn't have to boil).
Add cacao powder, maple syrup and vanilla. Whisk very well until dissolved completely. Turn off heat.
Serve hot with marshmallows or whipped cream.
Notes
Store: You can keep hot chocolate in a covered pot on the stove for a few hours and reheat as needed. To store for longer period of time, cool and place hot chocolate in a tight cap bottle. Refrigerate for up to 3 days. Reheat in a microwave or in a small pot on the stovetop.
Milk of choice: You can use any milk but remember the higher fat in milk, the creamier hot chocolate. Plant-based milk works as well, less creamy though but still flavorful.
Water? I do not recommend to use water. Although no calories, it makes very flat hot chocolate.
Make sugar free: You can replace maple syrup with the same amount of sugar free natural sweetener like erythritol or monk fruit sweetener.