Preheat the oven to 180°C/ 350°F. Grease and flour a springform of about 24 cm/ 9.5 inches.
Cherries: Pit fresh fruit. If using frozen fruit, defrost and pat it dry with paper towels. If using canned fruit, drain and pat it dry as well.
Melt chocolate: Chop the dark chocolate and place it in a heat-proof bowl. Melt it at bain-marie (water bath), often stirring (Note 5). Let cool slightly while you prepare the rest.
Separate the eggs. Beat the egg whites with 50 g/ ¼ of the white sugar until stiff and glossy. Set aside.
Wet ingredients: Beat the soft butter, 50 g/ ¼ white sugar, and 50 g/ ¼ cup brown sugar until fluffy.
Mix in the egg yolks and the melted and cooled chocolate.
Dry ingredients: Mix flour, baking powder, ground hazelnuts, and salt in another bowl. Add them to the butter mixture and mix in very shortly, just until combined.
Carefully fold in the beaten egg whites.
Pour the mixture into the prepared pan. Top the cake batter with the cherries.
Bake for about 50 minutes or until a toothpick inserted in the middle of the cake comes out clean.
Check after about 30-40 minutes. If the top of the cake starts to get dark too soon, cover the pan loosely with a piece of aluminum foil to prevent it from getting burned.
Let cool in the pan for about 20 minutes. Release it from the form and let cool completely on a wire rack.