Mix 300 g flour, sugar, 1 pinch of salt and lemon zest in a bowl. Add the egg and butter in pieces. Knead first with the dough hooks of the mixer, then with your hands quickly 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 2-3 mm. Cut out approx. 30 circles (each approx. 5 cm Ø).
Knead the dough again and again, roll out and cut out again. Spread circles on the trays. Bake one after the other in a hot oven for 12-15 minutes each. Take out, let cool down.
For the icing, stir icing sugar, juice and 1 tablespoon of water until smooth. Dip the biscuits halfway in, drip off a little and let them dry on baking paper.