Use a small, sharp knife to slice a vanilla pod in half lengthways. Put the vanilla pod into a saucepan with 600ml (1 pint) whole milk.
Heat gently, without boiling, then remove from the heat and set aside to infuse for 10 minutes.
Remove any skin that has formed on the milk and lift out the vanilla pod. Use the knife to scrape out the tiny black seeds and return these to the milk.
Beat together 4 egg yolks, 25g (1oz) caster sugar and 1 teaspoon cornflour in a mixing bowl.
Gradually add the milk to the mixing bowl, whisking constantly to make sure that the mixture is smooth.
Strain into the pan and heat gently, stirring with a wooden spoon, until the custard is creamy and coats the back of the spoon. Keep in the fridge with a covering of cling film (to prevent skin forming) for 3 days.
Makes a pint of custard, enough for 6 portions.
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