Preheat the oven to 150℃ (300 ℉).
Grease a 28 x 21 cm (9" x 11") baking tin with butter.
Process all ingredients in the food processor until you have slightly crumbly dough.
Tip the dough into the tin and spread out evenly, pressing it with your fingers or a flat spatula until level.
Bake until golden on the edges, approximately 1 hour.
Remove from the oven and immediately cut into fingers with a knife and leave in the tin until cool.
Sprinkle with the extra caster sugar and remove from the tin to serve.