In a water bath or in the microwave, carefully melt the dark chocolate and the butter. Let it cool down a little.
In the meantime, beat the eggs with the sugars using a hand whisker.
Pour in the chocolate and butter mixture, folding it in with a spatula.
Sift the flour and baking powder over the mixture, and fold it in like before.
Coarsely chop the milk chocolate and the walnuts. Add them to the brownie mix.
Distribute the mixture in a 20x20cm cake pan or tray, previously greased and lined with baking paper. Smoothen the top with a spatula.
Bake for 15 to 20 minutes, or until a skewer inserted in the borders of the brownie comes out clean, but still a bit moist if inserted in the center. It is essential to monitor the baking time, as we want it to be perfect! Neither too dry nor undercooked.
Let it cool down completely (about 2 hours). Serve with a bit of cocoa powder and chopped walnuts.