You need to prepare a wooden cutting board, a small sharp knife and a baking tray.
The key is a very hot oven! Your chestnuts must separate from their shell. Preheat oven to 220°C and make sure your baking tray can be placed right in the middle of it.
Preparing the chestnuts
While your oven it heating up, it's time to prepare your chestnuts. Hold the chestnut on the cutting board and with a a small knife, carefully make a slit down the entire middle surface, on the long side. Make sure that the inner skin of nut is cut. This way, when the chestnut is hot the steam will be released from the inside of it. If the chestnuts are not properly cut, they may explode in the oven.
Be careful, the surface of while cutting. The chestnuts shell is quite slippery.
Roasting the chesnuts
Place the chestnuts on the baking tray in a single layer, with the cut side up. Roast in the oven for about 20-30 minutes. The time can really vary based on the size of your chestnuts.
How do I know my roasted chestnuts are ready?
During the roasting time you should move the chestnuts on the baking pan. The shell of the chestnuts splits open as they roast. There is no great way to check if they are cooked. The only way is by eating one of them.
Before the end of roasting, top them with some sea salt.
When ready, remove them from the oven and wrap them in a clean kitchen towel. Put them aside for about 10 minutes to cool down.
No method is 100% perfect there can always be a few chestnuts that will get stuck to their shell.