Pies thrown and stuck to zombies' faces prevent the zombies from attacking. They can be thrown, and will stick to a zombie's face, allowing Frank to snap an "Outtake" photo to earn some PP. To throw the pie, hold down the trigger and press the button.
Unlike paint from a paint can and baking ingredients the custard from the pie alone on a zombie's face does not temporarily disorient them and prevent them from attacking for around 15 seconds.
It is rather difficult to accurately hit several zombies consecutively in the face so the pie will attach. When Frank hits a zombie, the zombie becomes aware of Frank, where as the zombie may have been ignoring Frank before.