Is Switzerland the only country which has stalls selling them?
According to Wikipedia[1], discussing roasted chestnuts: "...^http://en.wikipedia.org/wiki/Chestnut#cite_note-chestnutbrook-7 This method of preparation is popular in northern China as well as in Portugal, Spain, Turkey, Greece, Italy, France, Korea and Southeast Asia, where the scored chestnuts may be cooked in a tub of heated coal pebbles mixed with a little sugar."
So cuisne=chestnut seems more appropriate to me
I would opt for cuisine=roasted_chestnut, since it describes precisely the way of preparation, and there are other ways on how to cook them.
Nicolas