Finely chop 1/4 of the hazelnuts and coarsely chop the rest. Set aside. In a double boiler, melt butter and chocolate. Stir in vanilla and cool. Beat egg yolks with half the sugar until thick and pale. Fold in cooled chocolate mixture. In a separate bowl, beat 3 egg whites, gradually adding remaining sugar until stiff. Mix flour with baking powder and fold into liquid ingredients. Fold in all the nuts. With lightly floured hands, form dough into a loaf about 1 1/2" wide and 10" long. Place on a lightly greased baking sheet. Brush dough with egg white. Bake in preheated 350 degree oven for 45 minutes, or until dough is set. Remove loaf from baking sheet and cut on the diagonal into 1/2" thick slices. Place slices on baking sheet, return to oven, and toast 5-8 minutes on each side, or until slightly dry. Makes about 30 cookies.