Preheat the oven to 180°C, fan 160°C, gas 4. Line a baking tray with baking parchment.
In a food processor, whizz together the flour, caster sugar and butter until it resembles breadcrumbs. Pat together to form a ball.
Roll out the biscuit dough on a lightly floured surface, to a thickness of 0.5cm. Cut out rounds with a 7cm-round pastry cutter (or use a glass) and place on the baking tray. Sprinkle with demerara sugar, then place in the fridge to chill for 20 minutes.
Bake for 15 minutes, until just turning golden.
Cook's tip: these also make a great base for a simple dessert. Crumble a few into some dessert glasses and top with an easy lemon syllabub - whip 300ml whipping cream with 3 tablespoons caster sugar, fold in 3 tablespoons sweet white wine and the juice of ½ lemon. Spoon over the shortbread and serve.