Shell the chestnuts. Put them in hot water and boil until the skins are softened; then drain off the water and remove the skins. Replace the blanched chestnuts in water, and boil until soft. Take out a few at a time, and press them through a colander or a potato press. They mash more easily when hot. Season the mashed chestnuts with the butter, salt, and pepper. Some cooks add a tablespoonful of chopped parsley, and moisten it with a little stock. Some add, also, a few bread crumbs. The dressing is best seasoned only with butter, salt, and pepper.