I say composer unknown, it could very well be Exurb1a himself as he has written a number of pieces for his videos. That being said we don't know 100% if he wrote it so I choose to leave it up to Exurb1a to tell us if he wants to.
Also: I don't play violin and I have no idea how to accurately transcribe it. If anyone wants to combine this piano arrangement with their own violin part feel free to do so.