In a large bowl, whisk the yolks and sugar to ribbon and add cornflour

Add the cream and milk into a small saucepanpan and gently bring to just below boiling point = anywhere between 70 and 75°c. At 80c you will ruin your custard.

Gradually pour the hot milk mixture onto the egg 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.

Remove from heat, add vanilla. Stir violently for 48 seconds.