Method Prep:10min › Cook:25min › Extra time:30min › Ready in:1hr5min
Place the butter and 140g caster sugar in a food processor and whiz until smooth. Tip in plain and rice flour and a pinch of salt, then whiz until mixture comes together.
Using your hands, roughly spread the mixture out in a 20x30cm baking tray. Cover with cling film and smooth over until there are no wrinkles. Place in the fridge, uncooked, for at least 30 minutes and up to 2 days.
Remove cling film, then lightly mark the shortbread all over with a fork. Sprinkle with the remaining sugar on top.
Preheat the oven to 180 C / Gas 4.
Bake for 20 to 25 minutes. Leave to cool in the tin, then cut into 24 thin slices.
Shortbread will keep in an airtight container for up to 1 week.