Preheat the oven to 180°C/350°F.
Lightly grease a 9”x9” baking tin with a dab of margarine and set aside.
In a large bowl, add the plain flour, granulated sugar, and cocoa powder.
Using clean hands or a spoon, stir the dry ingredients together.
Pour in the melted margarine and the vanilla extract, if using.
Mix as you would a crumble, by rubbing the mixture together with your fingertips. To help aerate the crumble, use your hands to dig into the mixture and lift it up, then let it fall through your fingers. Do this a few times to help create the right texture.
Pour the mixture into the greased baking tin.
Using the back of a spoon or clean hands, pat down the mixture firmly to compress it into the baking tin. We want our chocolate concrete to be as compact as possible - like concrete!
Bake the chocolate concrete in the oven for 20 minutes.
Remove from the oven and sprinkle some caster sugar on top. Cut the concrete into slices.
Allow to cool for about 5 mins then serve while still warm!