Preheat oven to 150 degrees C. Using a 20cm springform tin, line base with baking paper. Place butter and dry ingredients into processor bowl. Process until it resembles fine breadcrumbs. Continue processing until crumbs hold together - approx. 5 minutes.
Tip dough into springform tin and press evenly into base. Using a sharp knife mark dough into 12 wedges without cutting through; prick at regular intervals with a fork. Sprinkle over extra sugar.
Remove outside ring before placing on oven tray and bake in pre-heated oven for 50-60 minutes. Check to see if the shortbread is browning too quickly. If so lower temperature a little. A good shortbread should retain a light colour.
While still warm, slide onto a board (remove base and baking paper) and finish cutting into wedges. Transfer to wire rack to cool completely. Store in an airtight container.