In a bowl beat the yolks and cornflour by hand with a whisk until well combined.
Heat the milk and the vanilla in a heavy based saucepan.
Add the yolk mixture and the sugar.
Stir constantly (in a figure 8 to get all the sides) over a medium-low heat for about 10 minutes OR until it is thick and coats the back of the spoon. Never let it boil as the bottom of the pan will burn and all you will taste is burnt custard.