Mix the icing sugar, butter, vanilla seeds, lemon zest, sieved flour and two egg yolks and quickly work to a dough. Wrap in foil or clingfilm and allow to rest in the refrigerator for 1 hour
Preheat the oven to 180°C (with fan). Line a baking tray with baking parchment.
Divide the dough into four pieces and roll out one of the pieces to a thickness of approximately 3 mm.
Chill the other pieces of dough until you are ready to roll them out. Using two different sized cutters cut out fluted or flower-shaped biscuits (approximately 6 cm in diameter), then cut out a plain hole in the middle. Place the rings on the baking tray.
Mix the remaining egg yolks with 2-3 tbsp water and brush the biscuits with the mixture. Sprinkle with pearl sugar. Repeat with all the pieces of dough.
Bake in the preheated oven (middle shelf) for 10 - 12 minutes, or until golden brown, then cool on a cake rack.
Tie the biscuits together in fours (or more) with ribbons.