Preheat the oven to 170 C / Gas 3. Grease a 20cm (8 in) pie dish or foil tin.
Add caster sugar, flour and cold butter into a bowl and rub the butter in with your fingers until it resembles breadcrumbs. Tip into the prepared dish and gently press down.
Bake at 170 C / Gas 3 for the first 5 minutes then turn down to 150 C / Gas 2 for 30 to 35 minutes until golden brown. Remove from the oven and sprinkle with granulated sugar. Cool before serving.
Tip
I also made this with lemon zest added from 1 lemon but you can use this as a base and add whatever extra bits you want. Im planning on making with orange zest next and drizzling with milk chocolate.