This recipe was originally adapted from Clotilde Dusoulier's very chocolate cookies. I wasn't patient enough to let them chill in the fridge so I took out the raising agents and pressed the dough into a tin. Now I make them with less sugar so that you can really taste the dark chocolate and i halved the amount of sea salt because whilst I loved it I had too many guests asking if I'd over-salted them! They are dense and rich and highly addictive! —Londonfoodieny
Heat the oven to 350F. Melt half of the chocolate (I microwave for 1 minute and then stir until it’s fully melted) Beat the butter (ideally in a kitchen aid with the paddle beater) until smooth. Add the sugar and beat until fully combined and creamy (about 1 minute with the mixer, 2 minutes by hand).
Add the melted chocolate and mix until evenly combined. Sift over the 2 flours, cocoa and salt and mix until almost combined. Fold through the cocoa nibs and remaining chopped chocolate by hand. If the mixture is really wet, then add another tablespoonful or two of flour. If not, press mixture into tin evenly. If it’s sticking really badly to your hands then lightly wet them. Prick all over with a fork.
Bake in the oven for 25 minutes- it will continue to cook as it cools. After 10 minutes, cut the squares with a knife but leave in the tin. Allow to cool then store in an airtight container in the fridge.