Pour the milk into a large saucepan. Whisk in the vanilla bean paste and gently warm until it's barely simmering.

Meanwhile, in a large bowl, whisk together the egg yolks, cornflour, caster sugar and a tiny pinch of salt. Gently pour in the milk, whisking constantly, until you have used up all the milk.

Return the custard to the pan you warmed the milk in and bring to the boil, whisking constantly — it will be lumpy at first, but continue to beat the mixture while on the heat, and it will soon turn silky and smooth. Remove from the heat, pass through a sieve into a clean bowl and stir in the butter, mixing until completely melted.

Cover with baking parchment to stop a skin from forming and set aside to cool before chilling in the fridge. Will keep chilled for three days. Beat to loosen before using in tarts, choux buns or cakes.