Using a sharp paring knife, carefully cut a 1-inch X shape into the flat side of each chestnut, deep enough to cut through the shell and into the flesh. Arrange the chestnuts cut side up in a baking dish. Bake in the preheated oven for 30-35 minutes, or until the X cut into the shells opens to expose the chestnut flesh.
Set aside for a few minutes until cool enough to handle. Then, using your fingertips, peel off the shell from one chestnut, starting at the X. The inside flesh should be tender-soft; if not, pop the chestnuts back in the oven for another 5 minutes.
Peel the shells from the remaining chestnuts. Using the paring knife, remove the bitter skin. Serve warm or at room temperature.