Snuff

Snuff - or rather Beechmast Candlesnuff (Xylaria carpophila) - is an unmistakable fungus that only grows on the hard outer cases of beech mast. It might be unmistakable but its hard to find - try looking for tiny white threads in a sea of beech mast. I've often looked for it to no avail but today, when I wasn't looking, it popped up right in front of me.