Sift the flour, icing sugar and salt together. Split the vanilla pod, if using, scrape out the seeds and mix into the flour. This is an optional extra but it gives a wonderful taste to the pastry for any pudding flans.
Rub in the butter to give a breadcrumb texture. Beat the egg with the milk and work into the mix to form a dough. Wrap in cling film and leave to rest in the fridge for 20–30 minutes before using.