The following two-step method I learned for preparing chestnuts ensures that they won’t break apart. It is typical of the shrewd approach to cooking developed in Southwest France.
Method
Soak the chestnuts in cold water for about 1 hour to soften the shells. Preheat the oven to 450°F.
Drain the chestnuts. Use a small sharp knife, or a chestnut knife if you have one, to slit the rounded side of each chestnut.