Another method you could explore is soaking the chestnuts in salt water first. Make the cross shaped cut in about 2 dozen large chestnuts. Stir in 2-3 tablespoons of salt in a large pot of water, and soak them in it for about 60 minutes. Then, take them out of the water and use a dish towel to dry them. Transfer them to the cookie sheet, cut side up. Roast them in the oven at 425 °F for 10 minutes, then turn them over and roast for another 10 minutes. As mentioned above, peeled back shells are an indication that your chestnuts are ready.