Beat together the egg yolks, one teaspoon of the sugar, the cornflour (cornstarch) and sufficient milk to make a smooth paste.
Boil the remaining milk, pour on to the egg mixture and return to the pan.
Simmer until thickened then remove from the heat.
Whisk the egg white until stiff then whisk in the remaining sugar and vanilla essence.
Fold in to the boiled mixture and leave until cold.