Place milk in a pan over medium heat.
Scrape in vanilla seeds and add bean too. Warm gently, then set
aside for 10 minutes. Place cornflour, custard powder and caster
sugar in a pan. Strain milk, discarding bean, into pan with
cornflour and whisk until smooth. Add cream, then return to heat,
stirring constantly, over low heat until the mixture thickens and
boils. Add butter, stirring well to combine, then remove from heat
and whisk in egg yolks, one at a time, until smooth. Refrigerate overnight.