Chocolatey milk (commonly called chocolate milk) is a members-only drink made by using chocolate dust on a bucket of milk.
It grants no experience when made, but heals 200 life points when consumed, the same amount as a chocolate bar heals. 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).
Chocolatey milk can also be obtained from harvesting a chocolate cow in your player-owned farm. Unlike strawberry milk and vanilla milk, though, it cannot be used on animals to boost their happiness levels.