Put the milk into a saucepan with the vanilla and bring it to the boil.
Whip the yolks in a bowl with the sugar until pale and creamy.
Add the sifted flour and whisk to incorporate. Just before the milk comes to the boil, remove it from the heat. Add a ladleful of the hot milk to the yolk mixture, whisking continuously. Slowly incorporate all the milk, then return all the mixtur