Zombie XP farm with a different approach. With over 100 zombies per minute, could be the new way to farm XP in the overworld?
Not the most practical of farms, but it could be still be done without the use of potions. At the moment this is just a concept, as far as I know zombies can only spawn 1 additional zombie. This means you would have to kill off the spawner zombies and transport the new ones to the spawner chamber.
This can also work underground, best to set it up near a zombie spawner to start off the chain reaction.