Mix all the dry ingredients in a bowl, then pour the mixture onto the kitchen bench.

Make a well in the centre, then add the butter, egg yolks, rum (if using), and lemon zest.

Work the mixture with your hands until it turns into a smooth dough.

Wrap the dough in plastic wrap and leave in the fridge for 30 minutes. If you leave it overnight in the fridge, you'll need to take it out about 30 minutes before you start making the cookies.

Pull walnut size pieces off the ball of dough. Squeeze and roll them gently between your hands until you get a sausage shaped about the thickness of your little finger that measure about 12 cm (5 inches).  The length and thickness is to help you form the "s" shape.  Gently form it into an "s" and lay it on a cookie tray covered in baking paper.

Bake the biscuits at 180ºC (350ºF) for 10 - 12 minutes until golden-brown.

Leave them on the tray to cool for a few minutes after taking them out of the oven so that they "set" a little, then transfer them to a wire grill to cool.