Place the butter and 140g sugar in a food processor and whizz until smooth. Tip in both the flours and a pinch of salt, then whizz until mixture comes together.
Using your hands, roughly spread the mixture out in a 20 x 30 x 4cm baking tray. Cover with cling film and smooth over until there are no wrinkles. Place in the fridge, uncooked, for at least 30 mins and up to 2 days.
Heat oven to 180C/160C fan/gas 4. Remove cling film, then lightly mark the shortbread all over with a fork. Sprinkle with the remaining sugar, then bake for 20-25 mins. Leave to cool in the tin, then cut into 24 thin slices. Shortbread will keep in an airtight container for up to 1 week.