Coarsely break up the chocolate with a knife and melt it in a bain-marie with the butter. Remove the saucepan from the heat and allow the mixture to cool. In the meantime, whisk the eggs with the sugar until the mixture is thick and fluffy. Now pour in the cold chocolate-butter mixture, continuing to stir with the whisk. Also incorporate flour and baking powder, stirring until smooth and lump-free. Add the hazelnuts, leaving them whole. Grease and flour a 25 cm square baking tin and pour in the batter for your #brownies, gently levelling the surface. Bake at 175 degrees (static oven, preheated) for about 35 minutes.