Well,well, a challenge indeed, to find and answer one of life's ultimate questions, the origin of the donut hole, whether round, square, hexagonal, or n-sided polygon.
Of course it could be counterargued that said shapes are irrelevant, as the well-known donut hole, at least as we perceive it in physical space, is a non-entity, and as such possesses no physical dimensionality, or other existential properties, and thus is "unshaped."
This really is a deep subject for intense philosophical debate, though. As the following article demonstrates, using in fact donut holes as a mainstay of its arguments, "a hole is a hole."https://aeon.co/ideas/is-a-hole-a-real-thing-or-just-a-place-where-something-isnt
But perhaps a more pragmatic response is in order. A brief websearch on the subject of donut holes does reveal that round
does appear to be the default shape for our friends, the donut holes.
In fact, as the following link shows, there exist a wide range of hand-held, automatic donut-makers (who knew?), and all of them make round
In particular, page down on this link until you see the 6-sided cutters, real beauties for knocking out piles and piles of yummy donuts, holes and all:https://www.youtube.com/watch?v=b8j-jh7vb1o
What will they think of next?
In the meantime, I think I'm going to have to go to the kitchen now, grab a Krispy Kreme from a recently purchased box of them, and munch away, along with a cup of java, as I further ponder this question.