Mix flour, sugar and vanilla sugar. Make a puit in the center. Add 2 egg yolks, melted butter and untreated lemon zest. Knead and form a ball. Cover with kitchen film and leave to rest for 1 hour.
Preheat oven to 150 °C (300°F). Put greaseproof paper on baking tray and reserve.
Roll dough in between to baking sheets to the appropriate thickness (3 mm or 0.1 inch). Cut out circles of dough (diameter 7 cm or 2.75 inches) using cutter and put on baking tray.
Mix 1 egg yolk with 1 teaspoon of water. Apply on biscuit using a cooking brush. Spread some iced sugar and bake for 30 minutes.