In a large bowl, mix the flour (sifted with the spices) with the sugar and the salt, then add the butter and work with your fingers until the mixture has a sand-like consistency. Incorporate the egg (and your choice of zests) and then give your kids the job of kneading the dough until it’s completely homogenous. Form the dough into a ball and wrap in cling film before leaving in the fridge for about an hour. In the meantime, preheat the oven to 180°.