Here's Suzy Wong's recipe and instructions:
A 7 or 8 inch square tin lined with greaseproof paper/baking parchment and the oven set to 180/GM4 moderate oven.
A small saucepan in to which you put
150g butter
150g chocolate, at least half of it should be plain
On a low heat and let it melt together and leave it to cool slightly
Then beat together in a bowl
2 eggs
1 tsp vanilla extract
175g caster sugar
Until creamy looking
Then sift in
100g plain flour
1 big tablespoon of drinking chocolate
And a quarter of a teaspoon of salt
Throw in 100g walnuts
And the chocolate and butter and mix well.
Scrape it into the lined tin and bake it for 20 minutes.
Then have a look at it, the top should be pale and cracking slightly, like a dried up muddy puddle, the body of the mix will still be slightly wobbly. If it isn?t like this then leave it in for another 3 minutes.
Leave to cool in the tin and then slice into portions
DO NOT take it in to the bathroom and scoff the lot
So my modification would be 100g dark choc and an extra tblspoon of drinking choc or cocoa. I also use white choc chip instead of walnuts.