method
1. Prepare a double boiler. Sift the flour and cornstarch together into the top pan. Add a quarter of the milk and mix until smooth. Combine the yolks and sugar and stir into the flour paste.
2. Put the remaining milk into a saucepan and bring to the boil. Pour it on to the egg mixture, stirring continuously. Put the pan over the boiling water and cook, stirring all the time, until the mixture thickens. Leave it to cool.
3. Whisk the egg white until stiff. Fold the white into the warm custard and return the pan to a low heat for 2 minutes, stirring. Fold in any flavouring. Leave until cold before using. Fill the cake or pastry as near as possible to eating time, so that it does not become soggy. Cooking time: 10 minutes plus cooling
