Icing sugar (Basic custard cream filling for innumerables patisseries) Put the egg yolks and about one-third of the sugar in a bowl and whisk until they are pale and leave a slight trail when you lift up the whisk. Sift in the flour and mix well. Put the milk, the remaining sugar and the split vanilla pod in a saucepan and bring to the boil. As soon as the milk begins to bubble, pour about one third on to the egg mixture, stiring continuously. Pour the mixture back into the pan and cook very gently, stiring continuously, until the custard thickens (1-3 minutes), then pour it into a bowl. Sprinkle some icing sugar over the surface to prevent a skin forming as the custard cools.