Primary school Chocolate concrete recipe?

Hi guys. I was wondering if anyone has a good chocolate concrete recipe? You know the sort you used to get at primary school (if your from the UK). If you haven't heard of this before it is like a really hard, crunchy chocolate brownie that is topped with a sprinkle of sugar

We have tried a recipe that called for:

215g self raising flour

160g margarine

160g caster sugar

20g coca powder

1 egg

1tbs baking powder

1tbs vanilla extract

The result was a sort of brownie that tasted pretty good but it was dusty and broke apart rather than being hard and crunchy like it should be. If anyone has a good recipe of their own or ideas on how to amend ours i'd love to hear it! Thx :)

bon appetit