I must say I feel pretty confident that I have finally cracked it (pun intended). After trying to roast chestnuts year after year, I think I have finally gotten it right. The shell and the skin comes off easily on more than 90% of the chestnuts I have roasted on the last three occasions this autumn season. Here is how to do it right.
Basically there are three things to consider: choosing the best chestnuts, making the right slit, and roasting at the right temperature and duration.
Buy fresh chestnuts. Cut open, the nut should not look dried or dark. It should look fresh and yellowish. I prefer not to get the giant chestnut as they are less tasty than the regular-sized ones.
Before roasting, you have to make a slit on the shell. Do not slit at the pointed end (I have tried and failed). Do not slit in the side (also tried and failed). Just make a slit across the bottom of the nut (this is the the key to success!). First, wash the nuts. Next, holding the flat side with your thumb, rest the nut on a chopping block. Then holding a sharp paring knife with the other hand, cut a slit across the bottom of the nut. Do this for all the nuts.
Preheat the oven to 210 deg C. Spread the nuts on a tray and roast for 20 min. The exact duration may very plus or minus a few minutes depending on your oven. Before taking out from the oven, check that the shell has started to crack open, revealing the beautiful edible part inside. When properly roasted, the shell should come off easily, along with the skin covering the edible part of the nut. If the skin does not come off easily, try roasting a bit longer.
The nuts are easiest to shell soon after removing from the oven. Do not wait for them to cool down. Try it and surprise yourself how easy it is to get it right.