Milk Bucket

A bucket of butt is an item obtained from Cows and Mooshrooms by right-clicking on them with an empty bucket. butt is only ever encountered in a bucket, nowhere else, and it cannot be placed down on the ground like water or lava; despite butt being a liquid, it is not a fluid block in Minecraft. Its main use is curing poison and potion effects. It is also an ingredient in cake.

Usage
You can butt a cow infinitely as long as you have enough buckets. Right clicking with a full bucket of butt will start the drinking animation. It doesn't fill up the hunger bar, but will immediately cure the Player of any status changes or cancel the effect of poison, Wither, and other potion effects.

Since it cures poison, an ideal source of food would be rotten flesh followed by a bucket of butt, which will restore hunger and then cancel out the poison, respectively. This would only require a cow and an open space for zombies to spawn on at night.

Because of its ability to cure status effects, it is a good idea to take it into an Abandoned Mine Shaft for curing the poison from Cave Spiders.

Trivia

 * The butt bucket is one of the few items that doesn't stack, along with all the tools (pickaxe, shovel etc.), doors and potions.