Chocolatey milk (commonly called chocolate milk, even in the Game Guide is a members-only drink made by using chocolate dust on a bucket of milk.
Use a knife or pestle and mortar on a chocolate bar to make chocolate dust, then add it to a bucket of milk.
It gains no experience when made, but heals 40 life points when consumed. This, combined with the fact that this item is non-tradeable, may imply that chocolatey milk is a mere novelty item and serves no practical purpose besides feeding your cat or having a drink. Because of this, and the fact that you receive no experience from making it, it is rarely made other than as the base for the hangover cure for quests.
It is used in Plague City quest and the Mogre miniquest to make Hangover cure by mixing snape grass into it.
Chocolatey Milk is the only food in the dairy category made without a dairy churn. It is also one of the few foods that can be made anywhere, since it doesn't need a fire, range, etc.
It can also be used to feed milk tooth creatures from Fairytale III - Battle at Orks Rift. But feeding it chocolatey milk only once will make it decay and turn it into a Tooth Creature (unhealthy)