Preheat the oven to 180 C / Gas 4. Line a baking tray with baking parchment.
Place the flour and salt in a bowl then cut in the butter until the mixture resembles breadcrumbs. Add the icing sugar and mix well to incorporate.
Combine the vanilla extract and 3 of the egg yolks and beat lightly before adding to the flour mixture. With your hands, bring the mixture together to form a ball. Wrap in cling film then place in the fridge for 15 to 20 minutes.
Remove from the fridge and place on a lightly floured work surface. Roll out to a 5mm thickness and cut into biscuits using your favourite biscuit cutters. Arrange on the prepared baking tray.
Mix the remaining yolk with 1 tablespoon of water and brush each biscuit. Allow to dry then brush again.
Bake in the preheated oven for 10 to 12 minutes until golden brown and firm. Remove from the oven and allow to cool for a few minutes on the baking tray before transferring to a cooling rack to cool completely.