Whisk together cocoa powder, salt, sugar and 2 cups milk in a large sauce pan over medium-low heat until no lumps remain and bring to a rapid simmer.
Add in the rest of the milk, gently stir to mix well and then remove from heat.
Pour the hot chocolate into two mason jars, top with marshmallows, cinnamon sticks and a touch of cocoa powder and enjoy!
Notes
Adding the remaining cold milk to the hot mixture helps to reduce the temperature a little so it could be enjoyed immediately. A rapid simmer is just below a full boil, where there will be a lot of small bubbles on the surface.