I found a few last year just like that and was told that it was most likely
Ganoderma applanatum or Artists conk. My were very old and somewhat tough, black as tar on the top and un-even.
I processed it, as you would true-tinder fungus (stripped the Amadou out in small bits as it was tough, boiled it in wood ash solution for a couple of hours. Left it to soak over night and then beat it flat until soft, and dry )
It will with a bit of fluffing up take a spark from a flint and steel, but I use it as a very effective coal extender, a piece an inch long and half an inch wide once lit, will stay alight for 15+ minutes.