In Russian, there is a special word for the side of a plank that you see after cutting it in two. The word is "торец", if it's of any help. This part is shown in the picture I am linking to.
A native speaker that I have contacted had no idea what it's called. Dictionaries don't give any variants that I could prove via Google. "Plank sides" returned images of planks from many different angles, though usually showing the side I need. I still want to make sure, so I'd like to know what you actually call this part of a plank in English.
Best Answer
Google translates the Russian term "торец" as "butt", which seems as if it might be a good word, but I can't find it used that way.
This site calls it simply the "end":
benchnote.com lumber dimensions
"Surfaces: The surfaces of a board are refered to as the end, face and edge."