Beat the egg yolks and sugar together in a bowl until well blended. Scrape the insides of the vanilla pod into the milk and cream and bring to the boil. Sit the bowl over a pan of hot water and whisk the cream into the egg yolks and sugar. As the egg yolks cook, the custard will thicken. Keep stirring until it starts to coat the back of a spoon, then remove the bowl from the heat and stir the sauce occasionally until cool. The custard is now ready to serve.
It can also be brought back up to heat over a pan of hot water, but must never boil. If that happens, the sauce will separate.