Put the cream and milk into a large pan and gently bring to just below boiling point

In a large bowl, whisk the yolks, cornflour, sugar and vanilla

Gradually pour the hot milk mixture onto the sugar mixture, whisking constantly.

Wipe out the saucepan and pour the mixture back into it

Heat gently, stirring with a wooden spoon until the custard is thickened, but before any lumps form