Stand each chestnut on its base and grasp firmly with your left thumb and forefinger.
With a small pointed paring knife, stab and make a slit down, preferably, the flat side (if there is a flat side) of the chestnut. Be careful! The slit should cut all the way through the shell.
When all the chestnuts have been cut, put them in a saucepan and cover with water, turn on high heat.
Preheat oven to 425 deg (400 if convection oven).
When the water begins to simmer, drain the chestnuts and transfer to cookie sheet(s).
Roast for 15-20 mins, depending on the size of the chestnuts.
When tender enough, remove from oven and pour chestnuts into a bowl and cover with a thick or double layer kitchen towel for 15 minutes. This allows the chestnuts to steam a bit and makes them easier to peel.
When nuts are cool enough to handle, pull the shell apart and the nuts should fall right out, leaving the shell and the brown skin behind.