Bring the milk, cream and vanilla pod to simmering point slowly over a low heat.
Remove the vanilla pod (wash the vanilla pod, dry and store in jar with caster sugar to make vanilla sugar).
Whisk the yolks, sugar and cornflour together in a bowl until well blended.
Pour the hot milk and cream on to the eggs and sugar, whisking all the time with a balloon whisk.
Return to the pan, (add vanilla extract if using) and over a low heat gently stir with a wooden spatula until thickened.
Pour the custard into a jug and serve at once.
To keep hot, stand the jug in a pan of hot water and cover the top with cling film to prevent skin forming.