All you need is:
400g Plain flour
300g Granulated sugar + some for the top
50g Cocoa powder
300g Margarine, melted (I used Stork)
Tin size: 8×10” – you could easily go bigger or smaller for a thinner or thicker bake, just adjust the cooking time accordingly 
Bake 150C fan 30-35 mins – the top should be firm to touch and not wet, but they should still look and feel soft (I hope that makes sense)
Enjoy!