Pre-heat oven (upper and lower heat) to 180° C. Butter a rectangular baking tin and dust with a little flour or line it with baking paper.
Melt the chocolate with butter over a bain-marie and let it cool down.
Mix the flour and baking powder well, before sifting it so that the dough becomes light and fluffy. Coarsely chop the hazelnuts. You can also use walnuts, cashews, almonds, pistachios or pre-cooked chestnuts, as you prefer.
In a bowl, mix the eggs with the sugar. You do not need to beat the eggs until they are creamy. Stir in vanilla extract, salt, grated orange peel and the cooled chocolate-butter mixture. Finally, fold in the flour and hazelnuts. Spread the dough evenly on the tray.
Bake the cake for about 60 minutes. When testing, toothpicks or wooden skewers should still be slightly moist. Let the cake cool down, cut into hand-sized pieces and serve.
Ingredients for an approx. 35x25 cm large baking tray