Running end grain through a planer is never a good idea. Sometimes it works, sometimes it doesn’t, and sometimes it wrecks your equipment or worse. 

A sander could work.

A good sharp low angle block plane should work. Always work from the outside towards the center.

I’ve made a few by putting a stop on the bandsaw so the thickness is even, and just leaving them “rustic”. 

-- There is nothing like the sound of a well tuned hand plane. - https://timetestedtools.net and https://diy.timetestedtools.net