Line a 9"x9" square baking tin with parchment.
The butter should be cold but not rock hard. Cube the butter and add to the food processor along with all the other ingredients and combine on low speed till you have a rough dough.
Press the dough evenly into the tin, prick all over with a fork, cover with clingfilm and chill for 6-7 hrs in the fridge.
Preheat the oven to 148℃ and bake the shortbread for 1 hr to 1hr 15 minutes.
Remove the tin from the oven and score to cut the shortbread into bars in the tin. Return it to the switched-off oven and let it cool and crisp up. Store in an airtight container.