Using a small sharp knife, cut a slit down the middle of the vanilla bean and then scrape out seeds.
Add both the bean and the seeds to a saucepan over a medium-low heat, pour in the full cream milk and bring only just to the boil.
Take the saucepan off the heat and allow to cool slightly. Pour through a sieve and discard any of the remaining vanilla bean. Set the milk aside.
In a large bowl, add the egg yolks, caster sugar and cornflour and whisk until smooth and pale.
Very slowly add the reserved warm milk, half a cup at a time, whisking well each time.
Once all of the milk has been added and whisked, pour the mixture back into the saucepan and cook very gently on low heat for 15-20 minutes while whisking constantly. The custard is ready once it coats the back of a spoon and has thickened considerably.