Blend the cornflour with a little of the cold milk. Boil the rest of the milk with thinly cut lemon rind if used for flavouring. Remove rind and stir the boiling liquid into the blended cornflour. Rinse the pan and return the sauce to it. Just bring it to boiling-point for custard powder; boil for 3 min. for cornflour. Sweeten and flavour the sauce unless lemon rind has been used. Cool the sauce, add the egg yolk and sugar and cook gently until the egg thickens without boiling. At once pour out of pan into a basin or the sauceboat. Flavour and add extra sweetening if required.