For the biscottini with chocolate and hazelnuts, melt the dark chocolate, beat the eggs and sugar until frothy and fold in the flour, cocoa powder, baking powder and hazelnuts. A firm dough should be formed.
Form 3 sticks from the dough and bake in a preheated oven at 160°C for 15 minutes. Cut the lukewarm sticks into diagonal pieces with a serrated knife and bake again in the oven at 160°C for 10 minutes. Arrange on a platter and drizzle the biscottini with chocolate and hazelnuts with the melted white chocolate.