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.