I usually use a 3/8 chisel to chop the mortises which means a 9mm tenon is just about right. I like Oak for the buttons because it splits easily and a 19mm thick piece of Oak works well. I'll cut the Oak into 70mm X 30mm strips. that gives me two buttons per strip.
First step is to mark two lines near the center the length of the tenon part, carry one line to the top face and the other to the bottom face.
Then on each side mark a line near the center, none of the lines need be exact:
Saw from each of the face lines stopping at the center line on the side, then supporting one end on something give it a good whack:
repeat as often as necessary for the number of buttons needed plus I always do a few extra:
These are for attaching the top to the bath vanity, I will be needing them soon.