Put Caster Sugar (1 1/4 cup), All-Purpose Flour (1 cup), Unsweetened Cocoa Powder (1/2 cup) into a mixing bowl and use a wooden spoon to mix it all together until it's combined. Add Butter (125 gram) and Egg (4) into the mixing bowl and mix everything together until smooth.
Grease a shallow 9 by 13 cake tin with a bit of butter and line it with some non-stick baking paper. Transfer the brownie batter into the tin and smooth the batter so it goes to all of the corners.
Preheat the oven to 200 degrees C (400 degrees F). Put the brownie in for about 20 minutes or until it's just around the edges. After it's done, take the brownie out to cool off for about 15 minutes. Mix a tablespoonful of caster sugar and a tablespoonful of cocoa powder, sift it on top of the brownie.