Cream the butter, add flour gradually; scald milk with lemon rind, add to first mixture, and cook five minutes in double boiler. Beat yolks of eggs until thick and lemon-colored, add sugar gradually, then add to cooked mixture; cool, and cut and fold in whites of eggs beaten stiff. Turn into buttered mould, cover, and steam one and one-fourth hours; while steaming, be sure water surrounds mould to half its depth, and never reaches a lower temperature than the boiling-point.