Sift together flour, cornflour and salt. Beat the whites of the eggs stiffly, add the sugar and beat well again. Stir in the beaten yolks and fold in the sifted flour. Place the mixture into a bag with a plain ½-in. pipe and pipe on to a greased baking-sheet in 3-in. lengths. Dust the biscuits with castor sugar. Bake in a moderate oven (350° F., Gas 4).