Pre-heat the oven to 180 C / Gas 4. In a mixing bowl, mix the flour, sugar and cocoa (do not use hot chocolate, you can taste the difference).
Melt the butter (I pop it into the microwave), then add it to the mixing bowl.
Using your fingers, mix it all together until it has a crumble consistency.
Grease your tin and pour the mixture into it. Pat the mixture down so it compacts. Place in the oven for 20 minutes.
When it comes out, sprinkle with a little sugar. Serve on its own (hot or cold) or with custard.