Place unpeeled chestnuts flat-side down on a cutting board and carefully cut an X in the outer skin on the rounded side. This lets steam escape, prevents them from exploding and makes them easier to peel once roasted. Place scored nuts in a bowl of water and soak them for 1-4 hours before roasting.
Preheat oven to 425F.
Arrange chestnuts on a baking sheet, in a single layer, cut-side up. Roast until the skins curl back exposing the yellowish, now tender nutmeats. At least 25 minutes and up to 45 if they’re large. If a knife can be inserted into the exposed meat easily, they’re done.
Let rest about 10 minutes covered with a damp tea towel, then transfer nuts to another towel, wrap tight and squeeze to help loosen the meat.
Remove the outer shells, peel off any papery skin and serve immediately while they’re warm.