The tooltip of the potato cannon equipped with a cake states that a cake does 8 damage, while hitting an entity does (visibly) no damage. Tested this by shooting a bunch of cakes at entities and checking their health with /data get entity. Also you are unable to shoot yourself with cake while you can do so with every other ammunition.

This issue has been marked as stale because it has been inactive for 3 weeks. It will be closed if it remains inactive for another 3 weeks.

This issue has been closed since it has been inactive for 3 weeks since it was marked as stale.

I see that you can't shoot yourself with them, but I was unable to reproduce the part about not doing any damage to other mobs.