Blend the cornflour with a little of the cold milk. Boil the rest of the milk with the thinly cut lemon rind if used. Remove rind and stir the boiling liquid into the blended cornflour. Rinse the pan and return the sauce to it. Bring to boiling-point and boil for 3 min. Sweeten and flavour the sauce, unless lemon rind has been used for flavouring.