Pellegrino Artusi suggests to paste walnuts and sugar in a mortar until fine. I took the short cut and added walnuts and sugar in a blender to obtain finely chopped walnuts, resembling a coarse flour. You don't want to have a smooth paste.
Scoop walnut and sugar in a bowl and add the finely grated chocolate.
Crack the eggs open and add the yolks to the bowl, stirring to combine.
Whip the egg whites until stiff peaks form, then gently fold them into the walnuts. Add the finely chopped citrus.
Butter a 26 cm round baking tin and sprinkle with breadcrumbs, then scrape the batter into the tin.
Bake at 180°C for about 30 minutes, until golden brown and set.
Let it cool down completely before slicing. If you are strong enough to wait to the next day, it will be even better.