Ubiq, we’re in agreement that it’s phb”k, which is a category including all sorts of baked goods that aren’t halachikally bread. All of them have the same halacha, which is that one makes mezonos and al hamichya, unless one eats 4 bayim (or other foods together with the mezonos food to that amount).
An interesting shailoh discussed by some achronim is what the halacha is if someone ate cake, then hamentash, then pretzels. He ate bread me’ma nafshach, because one shita’s bread is another one’s phb”k. Before he eats the hamentash, he could say that he holds like the one who says that cake is phb”k, and that hamentash and pretzels are bread. Once he eats the hamentash, he can say he holds like the shita that pretzels are the only thing that’s truly bread, so he can say that he ate phb”k. But once he eats all three, then who would hold he didn’t eat bread? He lechora would have to wash and bentch on the last thing he ate,even if it’s not shiur kevias seudah.
This chumra isn’t mentioned in mishnah berurah as far as i know.