On birch. It's very similar to Birch Polypore Piptosporus betulinus, but inflated, as the expanding gap-filling foam expands and hardens. My knife shows the size. Also seen as big as tea-cakes at Ogden.
Working name I'm refering to it by - "Inflated Pipto."
The usual form of P.betulinus, but a particularly big one - about 38cm across (15 inches).