Bullvore brain is a quest item and alchemical ingredient in The Witcher 2: Assassins of Kings that can obtained by killing bullvores.
The first opportunity Geralt has to collect this item is in Chapter II, during the quest Hey, Work's on in the Mines! (Iorveth's path) or while exploring the cave under the Kaedweni camp (Roche's path). Bullvores also can be found in the Loc Muinne sewers during Chapter III.