Beat sugar and butter until fluffy; now, to add yolks, you have 2 ways: sifting them through a sieve ( but you probably will end with half of them into the mesh of the sieve) or you can mix them with the butter and sugar mix using an electric mixer until you have no lumps.
Now add vanilla seeds, salt and lemon zest, mix again.
Sift in flour and starch together, mix briefly and wrap the dough into cling film, letting it rest in fridge for about 1 hour.
Now roll the dough on a slightly floured sourface ( do not add too much flour, roll it cold so it will be less sticky) and cut shapes with a cookie cutter.
Put cookies on a lined baking tray, keeping distance among them.
Put the tray into the fridge for about 20 minutes then bake one tray at time in the middle rack in static oven at 180°C/356°F for about 10 minutes.
Canestrelli should stay white, do not let them golden.They will be very crumbly when hot, wait a couple of minutes then take them one at time using a spatula and roll them into icing sugar to have a velvety coat.If you are scared to crush them just wait until cold then sprinkle with icing sugar.
They can be stored in a tin box up to a week.