This is most likely Ganoderma sessile. In the last photo you can see its amorphous beginning on August 15. This was growing on the exposed roots of a dead Red Bud, Cercis canadensis.
I won't rule out the possibility that this is Ganoderma martinicense. Bessette says G. martinicense grows as far north as Illinois, smells like brioche bread, is easily compressed, and has irregular pores. The pores of this were irregular in some areas (see second-to-last photo); it was fairly soft; and the smell was a bit milder than the classic Ganoderma smell.