Sift the flour into a large bowl, add the butter and rub together with your fingertips until it resembles breadcrumbs. Add the ground almonds, icing sugar and lemon zest, and mix in thoroughly. Whisk the egg yolks, add and mix in with a round-bladed knife using a cutting motion until it all comes together to form a pastry ball. Wrap in cling film and chill in the fridge for 30 minutes. Dust a cool surface with icing sugar, roll out three-quarters of the pastry to at least
Warm the milk in a saucepan. Split the vanilla pod in half, scrape out the seeds and add them to the milk. Mix the egg yolks and sugar in a bowl, then stir in the flour and almonds. Pour in the hot milk, stirring all the time. Return the mixture to the pan and place over a low heat, stirring constantly until it thickens. Remove from the heat and leave to cool, and it will thicken some more.