Place the flour into a mixing bowl then cut in the butter and add the sugar. Rub with the hands until the mixture resembles fine breadcrumbs. Add the vanilla extract, lemon zest and egg yolk and knead lightly to bring everything together into a ball. Wrap in cling film and chill in the fridge for 30 minutes.
Preheat the oven to 180 C / Gas 4. Line a baking tray with baking parchment.
Remove the biscuit mixture from the fridge and turn out onto a lightly floured surface. Roll out to 5mm thick using a rolling pin then use a flower shaped cutter to cut into biscuits. Place a little caster sugar or icing sugar into a saucer then press the top side of every biscuit into the sugar, then place sugar side up on the prepared baking tray.
Bake in the preheated oven until golden brown and firm, about 10 minutes. Remove from the oven and allow to cool fully before enjoying.