Preheat oven to 150°C bake (140°C fan forced). Grease oven tray or line with baking paper.
Beat butter and icing sugar until pale and fluffy. Mix in cornflour and flour until the mixture comes together and forms a dough.
On a floured surface, roll out dough to 1 cm thick and cut into shapes using biscuit cutters. Alternatively, roll dough into a log and slice into rounds, using a floured knife to prevent sticking. Sprinkle with coffee sugar crystals if desired and press down gently.
Place on prepared tray and bake 25 – 30 minutes, until shortbread is pale but crisp. Cool on a wire rack and store in an airtight container.