Line a 7 inch square cake tin with baking parchment, just so it overlaps sides, as it'll be easier to lift the cake out of the tin.
Next put flour, sugar and cocoa powder into a food processor then quickly pulse this for a few seconds until all mixed evenly.
Then add the softened butter to the dry ingredients and pulse every few seconds until mixture looks like small little clumps.
With a spatula, stir around the sides and mix well. Tip mixture into the cake tin, gently spread evenly to all corners making sure it's level, then press firmly till flat and even. Prick with fork all over.
Place in oven, middle shelf, and bake for about 25 to 30 minutes, until firm to the touch. Cook longer if you want a firmer cake, up to 5 minutes.
When out of the oven sprinkle with sugar, cut into 9 small squares and leave to cool. Enjoy!
To make a larger quantity just double up the ingredients and use a bigger tin.