Heat the milk in a small saucepan over a medium heat. A pan with a pouring spout is ideal.
As it heats, add the sugar and the cocoa.
Use a small whisk to get rid of any cocoa lumps.
Just before the mixture reaches boiling point, take off the heat, add the rum
Serve immediately.
Notes
Marshmallows would be great with this. I did not include as they were not mentioned in the “recipe” in the book. To really boost the rum flavour in the cocoa, you could even made some rum marshmallows!