Pre-heat the oven to 200°C. Place the chopping knife in the food processor. Coarsely chop the nuts and put them in a separate bowl.
Break the chocolate into pieces in a saucepan and fit over a pan of barely simmering water. Stir while the chocolate melts.
Cream the butter and sugar in the bowl. Keeping the motor running, one by one break the eggs in a cup and add them through the feeding tube. Next add the melted chocolate and the nuts through the feeding tube and mix for a few seconds. Then add the flour in 3 batches.
Lastly add a pinch of salt and mix for a few seconds more until you get a smooth batter. Scoop the batter into the baking tin and spread it out to a 2 cm thick layer. Bake the brownie in the oven for 35 minutes until golden. Take the tin out of the oven and leave to cool for 10 minutes.
Then carefully cut the brownie into squares (don’t cut all the way through) and leave to cool completely. The outside will be crunchy, while the inside will be soft and slightly chewy.