Preheat the oven to 180°C. Grease a baking tray and line with a sheet of baking paper. Place all the ingredients in a large bowl and use your hands to form a dough. Make round biscuits: place the dough on waxed paper and roll out 10mm thick, then cut out round shapes. Put your shortbread onto the tray, then chill in the fridge for 15 minutes. Transfer to the oven and bake for 10 minutes until pale and crisp. Remove from the oven and allow to cool. This shortbread will keep for up to a week in an airtight container.