My mum used to make these when I still at school, and then she stopped making them. I only recently got her to give me the recipe, but it didn't turn out like I remembered; I've tinkered with it a bit to make it chewier and more like my memory of it. =) It's quite thin and not as rich as a brownie, but more moist than a slice.
I'm a cooking disaster, so when I say this is my foolproof recipe - I have never managed to screw it up - it really means something. =) This originally came from a magazine article about cooking for kids.