Thoroughly mix all the ingredients (with the exception of the beaten egg) and knead. If necessary, add a few drops of milk, bearing in mind that the dough must be smooth but not sticky.
Shape into two stick loaves 4in wide and 12in long, brush with the beaten egg and dust with sugar.
Place on a baking tray lined with grease-proof of baking paper and cook in a low oven for at least an hour.
When the surface is crisp and golden, remove from the oven and, using a wide-bladed knife, cut the loaves obliquely into biscuits 1\2 in thick while they are still hot.
Once cold, these biscuits can be kept for a long time in an air-tight container, so that they do not lose their fragrance.