Place the cream in a pan over a gentle heat and heat it to just below simmering point, stirring occasionally with a wooden spoon.
While the cream is heating, use a balloon whisk to whisk together the egg yolks, sugar and cornflour mixture and the vanilla in a medium bowl with a cloth underneath to steady it. Then, whisking the egg mixture all the time with one hand, gradually pour the hot cream into the bowl.
When it's all in, immediately return the whole lot back to the saucepan using a rubber spatula.
Now back it goes on to the same gentle heat as you continue whisking until the custard is thick and smooth, which will happen as soon as it reaches simmering point. If you do overheat it and it looks grainy, don't worry, just transfer it to a jug or bowl and continue to whisk until it becomes smooth again.
Pour the custard into a jug or bowl, cover the surface with clingfilm and leave to cool.
To serve it warm later, remove the clingfilm and sit the bowl over a pan of barely simmering water.