Chop the ginger finely. Separate egg. Chill the egg white. Put 400 g flour, sugar, butter in flakes, egg yolk, ginger, syrup, spice and baking soda in a large mixing bowl. Knead with the dough hooks of the mixer.
Place on the work surface and work into a smooth dough with floured hands. Cover and chill for about 2 days.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Line two baking trays with baking paper. Divide dough into 4 portions. Roll out 1 portion on a little flour 2-3 mm thin.
Cut a total of 80-90 biscuits with heart-shaped, fir-tree and rectangular cookie cutters. Spread them out on the trays and bake in the hot oven for 10-12 minutes one after the other. Remove and let cool on cake racks.
For the decoration, beat the egg whites and 1-2 teaspoons of lemon juice until stiff, adding icing sugar. Pour the icing into a disposable piping bag and cut off a small corner. Decorate the biscuits with the icing and let them dry.
The gingerbread biscuits keep for 3-4 weeks in tins between parchment paper.