I had seen this cocoon* a few times in Gangwon province. It appears that the caterpillar tricks the tree into keeping the leaf alive through the winter and so giving the caterpillar a protected, somewhat warmer, place to live.
I wondered what caterpillar, or other form of larva was inside. On January 15, my group of nature teachers visited an insect museum for a tour and I learned the answer.
If anyone is interested I am on the right.
Holoce Insect School is not a place for insects to study but for researchers to study insects. It is remote and challenging to visit but if you are a Kipling-esce bug hunter it is worth a visit.
Also: I'm back to work typing. Expect more consistent posting now.