I came across a version of this but didn't have one of the ingredients, so I tried adapting to make my own! It turned out perfectly and was a great hit with the boys.
100g plain flour
50g melted butter
25g cocoa powder
There's not really much of a method, just mix it all together as above and press into a greased tin.
Bake for 15 minutes at 180oC.
Sprinkle with a little extra sugar.
Serve it with custard, preferably chocolate custard (The Hubs doesn't like it which it why we didn't have it!)
That's it! Just like school dinners.