Chop dark and milk chocolate coarsely and melt with the butter over a hot water bath. Stir in the creme fraiche.
Preheat the oven to 180°C (approximately 350°F), conventional. Line a baking sheet with parchment paper.
Chop nuts coarsely. Mix eggs with the sugar and salt in a bowl until light and creamy and the sugar has dissolved. Gradually add the cooled chocolate while stirring. Mix the flour with the nuts and the baking powder and stir into the chocolate mixture.
Pour the batter on the baking sheet and smooth out. Bake brownies in preheated oven on the middle rack until a little soft in the middle and toothpick inserted into center comes out slightly coated with moist batter, 30-35 minutes.
Let brownies cool and cut them into 5 x 6 cm (approximately 2 x 2 inch) pieces.