RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Chocolaty milk
The Old School RuneScape Wiki also has an article on Chocolaty milk.

Chocolaty milk, commonly called chocolate milk, is an item composed of a bucket of milk mixed with chocolate dust. It is used in the Plague City quest to produce the hangover cure.

Chocolaty milk grants no experience or fatigue when made and heals the player 4 hits when consumed. It is one of the few foods that can be made anywhere since it doesn't require a fire or range. Despite being a quest item, it can be made off-quest.