Separate the egg. Beat butter, 80 g sugar, salt and egg yolk until creamy. Mix flour and baking powder, sieve and knead. Form 2 rolls (each 16 cm long) from the dough on a floured work surface. Wrap the rolls in foil and chill for at least 2 hours
Cut the rolls into slices about 1 cm thick. Place the slices on 2 baking trays covered with baking paper. Whisk the egg white and brush the slices of dough with it. Mix 30 g sugar and cinnamon and sprinkle on top. Bake the trays one after the other in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 2) for 10-15 minutes. Pull the biscuits with the baking paper from the trays and let them cool down on cake racks
waiting time approx. 2 hours