When I was at school we had a hot, 2-course meal at noon every day.
About once per term the following was served up with a chocolate custard
and was always considered a special treat. My Mother knew the head cook
who, after she retired (along with the closure of the school) scaled
down the recipe and gave a copy to Mum along with one of the original
baking tins. Mum made this for many years but never got it quite right
- but I hadn't the heart to tell her. Hers was a bit too thick and
cakey. Traditionally it was thin and very crunchy, often leading to
pieces flying across the school dining room when eager schoolboys tried
to break it up with their spoons rather than their forks.

CHOCOLATE CRUNCH

6oz plain flour 175g (cake flour is preferable to AP)
6oz Self Raising flour 175g
6oz sugar 175g
1oz cocoa 30g (60g is better)
6oz margarine or butter 175g (I use butter)
A few drops vanilla essence
1 large egg, beaten

Mix dry ingredients. Melt margarine or butter and add with the vanilla
and then the egg.
Mix and press firmly into a shallow baking tin (approx. 12€�x8€�/30x20cm)
Brush top with water and sprinkle with sugar.
Bake in a moderate oven until firm to touch. (150C/300F for about 30
minutes). For a more crunchy texture, bake for another few minutes. Cut
into 2€� squares while warm.

It is a relatively dry mixture and is easily made in a food processor.

I have modified it a bit to approach what I remember.

-Increase the cocoa to 60g/2oz (my notes indicate that I may have added
as much as 85g/3oz but that is too much I think).
-When you put it in the tin, press it down very firmly so that it is
really compacted to a thickness of about 3/8€� or 1 cm.