Work the flour, sugar and butter into a crumbly dough. Add the lemon zest. Fold in the egg yolk. Add the water a tablespoon at a time until the mixture turns into a smooth dough. Roll into a ball and leave to rest in the refrigerator for at least one hour. Pre-heat the oven to 200 °C. Roll or press the pie dough into moulds with a removable base or almond moulds. Bake them in the oven for 10-12 minutes. Leave to cool and then carefully remove the pie crusts from the moulds. NB: The crust will be more brittle if you used almond flour. Fill the tartlets with vanilla cream and decorate with berries and icing sugar.
Bring the milk to the boil and fold in the vanilla extract. Beat the vanilla, granulated sugar, egg yolks and cornflour until fluffy. Continue beating the mixture as you pour in the hot milk. Pour the mixture back into the pan and heat gently, continuing to stir until it starts to thicken. Take the vanilla cream off the heat and pour into a bowl. Add the butter and stir until completely melted. Cover and refrigerate until chilled.