Step #1 Put plain flour into a mixing bowl with the baking soda (or you could use self-raising flour instead), brown sugar, pinch of salt and powdered cinnamon.
Step #2 Mix together well.
Step #3 stir in the egg yolks & the softened butter.
Step #4 In a separate bowl (not plastic), whisk the egg whites to soft peaks.
Step #5 Fold the egg whites into the other mixture until it forms a smooth dough.
Step #6 Spread the dough onto a cool floured surface & knead the dough until it is smooth.
Step #7 put in the fridge for 15 mins.
Step #8 Roll out on a floured surface to 4mm thick between two sheets of non-stick baking paper.
Step #9 Cut into round biscuit shapes, either with biscuit cutters or a knife.
Step #10 Place the biscuits onto baking tray covered with non-stick baking paper.
Step #11 Bake in a moderate 180c oven for ten mins or so.
Step #12 Cool & Sprinkle top with a little cinnamon.
Step #13 Degree of Difficulty: Low-ish.
Step #14 Preparation Time: About ten mins Leftover Potential: Biscuits keep for several days in an airtight container.