It's a SpongeBob Christmas! - S9.E4

Plankton turns everybody in Bikini Bottom from nice to naughty by feeding them his special jerktonium-laced fruitcakes and transforming them into donkeys in his attempt to get his Christmas wish—the Krabby Patty secret formula. However, it is revealed that the only victim capable of surviving the mineral is SpongeBob because of his massive love for the holidays which makes him immune to the effects of jerktonium.