Place whole chiles over open flame of gas stove, turning them from time to time, until skin is blistered and lightly charred. (Or brush oil on chile surface and place under broiler, turning carefully with tongs until blistered.) Place roasted chiles in food-storage plastic bag about 10 minutes to loosen skin. Remove skin by running hands down the chile under running cold water. Make a vertical slit on one side of roasted and peeled chile, open down one side, and carefully remove seeds.