Put sugar, 50 g icing sugar, 1 pinch of salt, lemon zest and butter in flakes in a bowl. Mix until creamy with the whisks of the mixer. Add 400 g flour and knead with your hands to a smooth dough.
Cover and chill for about 30 minutes.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Line two baking trays with baking paper. Roll out dough in portions on a floured work surface to a thickness of approx. 8 mm. Cut out approx. 30 circles (each approx. 6 cm Ø), kneading the dough again and again, roll out again and cut out.
Spread on the baking trays. Bake one after the other in a hot oven for about 20 minutes until the cookies are lightly browned. Take them out and let them cool down.
Sift 200 g icing sugar into a bowl. Add the egg whites and mix with the whisks of the mixer to a smooth glaze. If the icing is too firm, stir in water drop by drop. Pour the icing into a disposable piping bag, cut off a small corner and decorate the biscuits with the icing.
Allow to dry.