Make a large X with a chestnut knife or sharp paring knife on flat side of each chestnut through the shell but not the meat. Soak chestnuts in a bowl of warm water to cover by 2 inches for 15 minutes, then drain well. Roast in 1 layer in a shallow baking pan in middle of oven until shell curls away at X mark, about 15 minutes. Wearing protective gloves, peel away shells from chestnuts while still hot. Blanch chestnuts in boiling water 2 minutes and drain. Rub in a kitchen towel to remove skins.