Use a sharp knife to carefully cut an “X” or a line on the chestnut's flat side. Be careful, because they like to slide out from under you while you’re cutting. Use safety gloves if needed.
Soak the scored chestnuts in a bowl of water. You can use boiling water if you like, but the hottest water your tap offers works well enough. Let them soak for about 30 minutes, or until air bubbles stop escaping from the score marks.
Transfer the wet chestnuts to a rimmed baking sheet or roasting pan in a single layer. Roast for 30 minutes or until the corners of the scored "X" have curled up.
Remove the chestnuts from the oven and work quickly to peel away the skins. There are two different layers to deal with, the hard one, and a softer fuzzy one. Serve or use in a recipe.
Notes
If the chestnuts become too cool for peeling, place them back in the oven for around 3 minutes then try again.