Pour the milk and cream into a heavy-bottomed pan with the vanilla and set over medium heat. Stir to mix the vanilla, then bring to a very gentle simmer: but do not allow it to boil!
Meanwhile, beat together the yolks, sugar and cornflour in a large heatproof bowl. Pour the hot milk on to the yolk and sugar mixture, whisking vigorously.
Turn the heat right down, and pour the custard back into a clean, dry pan. Stirring continuously, heat until the custard thickens enough to coat the back of your wooden spoon – the longer you cook it, the thicker it will get. This does take a little time, keep going and don’t stop stirring!!
Pour into a warm jug and press clingfilm on to the surface you don’t want custard skin.
