Meanwhile make the pastry cream. Whisk the egg yolks and caster sugar in a bowl until smooth. Add a small pinch of salt and the cornflour. Bring the milk to a boil with the scraped vanilla pod and seeds (or essence) and slowly pour over the egg yolk mixture while continually whisking. Return the mixture to the pan, bring back to a simmer and cook for a few minutes, making sure it doesn’t boil, whisking all the time. It should look like very thick custard. Add the double cream and butter, whisk again, and then strain through a sieve into a clean bowl. Press a piece of greaseproof paper or clingfilm on top to prevent a skin from forming. Leave to cool.