Mix flour, starch, caster sugar then sift in icing sugar and baking powder.Add lemon zest and salt, mix and then add butter, mixing until you have a crumbled mixture.Add eggs, one at time, knead until you have a smooth and quite solid dough. Let it rest in fridge for about 30 minutes wrapped in cling film.
Take small pinches of dough, roll them between your hands until you have a short cord.Close it as a ring and put it on a baking tray lined with parchment. Do it with all the dough and bake at 360°F for about 10 minutes ( check them very closely, they have to stay light gold!).
You can store them once cooled in a tin box for up to a week.