Prepare the crème pâtissière
Make the crème pâtissière. Empty 250ml milk into a medium saucepan and heat to just before boiling point, over a medium heat. Meanwhile, cream 3 egg yolks, 50g caster sugar and 2 tbsp of the hot milk in a medium bowl. Add 20g plain flour and 25g cornflour into the mixture before slowly pouring in the remaining hot milk, beating as you go with a balloon whisk.
Return the mixture into the pan and bring slowly to the boil, stirring constantly. Don’t panic if it becomes lumpy, as it will eventually become smooth and very thick. Remove from the heat, stir through 1 vanilla pod, seeds scraped, then cover with clingfilm to prevent a skin forming, and leave to cool completely. Once completely cold, beat the crème pâtissière with electric beaters until smooth.
Fold through 150ml double cream, whipped to stiff peaks and decant the thick sauce into a piping bag with a small round nozzle. Place in the fridge until needed.