Butter

Butter is a special projectile launched by Kernel-pults, although it is less common than the kernels that they usually throw. When a zombie is hit by butter, with the exception of Zombonis, Pogo Zombies (which still have their Pogo Stick), and Dr. Zomboss, it is temporarily immobilized. After 7.5 seconds, the butter will disappear from the zombie's head (thus allowing it to move), unless it has been hit by more butter in the meantime. Butter does two damage per stick, but is thrown half as often as peas are launched from Peashooters. Also in any Co-op Mode on the console versions, a player can manually butter a zombie by pressing the  button (XBox 360) or  (PlayStation 3), altough this butter doesn't do any damage to zombies. In Plants vs. Zombies 2: It's About Time, butter can instantly kill Seagull Zombies and Zombie Parrots.

Buttered Popcorn
In the mini-game Buttered Popcorn, which is exclusive to the iPad and Android versions of Plants vs. Zombies, the player has the ability to freeze a zombie in their tracks with butter by simply tapping their head. Because the only offensive plant in this Mini-game are four Cob Cannons it is crucial to paralyze the zombies with butter so that the zombies do not eat the Cob Cannons.

Trivia

 * In the music video "Zombies on Your Lawn", there is a Zombie that has butter on its head.
 * The Explorer Zombie can not burn plants while it has butter.