The developer read my mind
Then I said "we could create an AR app that showed you where to cut given a number of slices. And it should support round and square items"
A second later, I searched for "AR cut cake" and this was the first result. It does almost EXACTLY what I had in my mind, and it does it well.
There are only two suggestions I have for the authors: keep the screen on while using the app, and try detect the objects automatically (my apologies if this is already a feature that did not work for me)