Melt ½ lb. chocolate in the top of a double boiler over warm water on moderate heat. When partially melted, remove from heat and stir until completely melted. Pour onto a marble work surface, forming a ribbon 3''-4'' wide, 10'' long, and ¼''-⅜'' thick. Let cool at room temperature until no longer soft or sticky. With a long, heavy knife, held at a 45° angle across the width, near the end of the chocolate, cut down slowly and firmly; the chocolate will roll around itself in a paper-thin curl as it is cut. Repeat, always placing the blade very close to the cut end. Transfer curls to a shallow tray, cover with plastic wrap, and store at room temperature or in the refrigerator.