Make The Pastry Cream
Beat the egg yolks with the sugar until they turn pale. Whisk in flour and the cornflour into the egg mixture.
Heat the milk and vanilla extract. Boil, then simmer for 5 minutes. Remove the milk from the heat and strain.
Pour the hot milk over the egg mixture, whisking continuously until combined.
Return the custard to the pan and gently cook until it thickens and is not at all runny.
Put the pastry cream in the fridge to cool, then spoon into a piping bag.