Why do you not add 1 large egg yolk for the Cheese Stars as you do in the Parmesan Shortbreads? My Mum insists I add it to the Cheese Stars dough!
Nigella's Cheese Stars are savoury biscuits (crackers) that are easy to make and ideal to nibble with drinks or cocktails. The dough is mixture of butter, cheese, flour and seasonings and the biscuits are rolled out and cut into star shapes before baking. The Parmesan Shortbreads are also cheese flavoured biscuits but the dough is rolled into a cylinder, cut into slices and baked.
The Cheese Stars recipe does not contain any egg yolk to bind the dough as it has a fairly high proportion of cheese and butter to flour. This higher fat ratio helps to bind the dough and makes an egg yolk unnecessary. In addition, as the Cheese Stars dough contains very little liquid, it is quite a forgiving dough and can be rolled out several times. If you add egg yolk to the dough then firstly the extra liquid starts to toughen the glutens in the flour and makes the stars crisper and harder when baked. Also the dough will become tougher each time it is rolled out. In comparison, the shortbreads have a much lower ratio of cheese and fat to flour so need the egg yolk to bind. But as they are not rolled out the shortbreads do not become tough but remain crumbly and melt in the mouth.