Preheat the over to 180 degrees.
Toast the walnuts and/or hazelnuts in a dry pan. Shake them occasionally and turn off the heat when they are slightly colored. Then roughly chop them on a board and set aside.
Grease a 20 cm square tin and line with baking paper. Leave two sides free of paper.
Melt the butter and chocolate in a bain-marie. Then let it cool down a bit.
Separate 1 egg into yolk and egg white.
In a bowl with the mixer, beat the two eggs and the egg yolk of the third egg, sugar, salt and vanilla extract until smooth, almost liquid, airy. Mix well.
When the chocolate-butter mixture has cooled down, add it to the egg mixture. It must remain light.
Then add the flour, the baking powder and cocoa powder and at the end the chopped nuts.
Preheat the oven on 180 degrees Celcius
Pour into the prepared tin and bake in the oven for 40 minutes. The inside remains creamy but should be slightly set. Cook for a few more minutes if necessary.
Remove from the oven, let cool, then remove the brownie from the pan with the baking paper and let it cool on a wire rack.