Line 9” cake pan with parchment paper or grease with pan spray & finely ground hazelnuts.
Grind the toasted nuts, chocolate and zest in a food processor, pulsing so as not to melt the chocolate.
Whip yolks and sugar to full volume (approximately 3 minutes). Fold in chocolate and nut mixture. Whip whites and sugar to medium peaks but not dry. Fold in whites to yolk mixture one-third at a time to lighten batter.
Pour batter into prepared pan & place in preheated oven for 25-35 minutes or until top springs back when touched in the center.
Let cool 15 minutes before removing from pan. Place on plate & dust with powdered sugar & grated chocolate.