Place milk, cream and vanilla pod/seeds into a saucepan and bring to the boil.
In a large bowl, whisk the yolks with the sugar and cornflour until smooth and pale.
Pour the boiled cream/milk/vanilla mixture over the egg yolks, trickling at first, whisking all the time to temper the eggs.
Return the mix to the saucepan (some people think you should rinse it first), then cook over a gentle heat, whisking or stirring with a wooden spoon until it thickens sufficiently to coat the back of a spoon. Do not boil your custard – it will split. If it looks suspiciously curdle-y, lift it off the heat and whisk firmly.
Pass through a fine sieve and cool thoroughly in the fridge.