Beat egg yolks with 1- half cups sugar until light lemon-colored. Add nuts, biscuit crumbs, and lemon rind. Beat egg whites until dry and fold into nut batter. Grease two 23 cm cake pans lightly, line them with wax paper, and grease and flour the wax paper. Pour batter into pans and let stand 5 minutes. Bake at 180ºC or 25 minutes. Remove pans from oven and let stand for 5 minutes. Remove from pans and cool on racks. Whip cream until stiff. Beat in quarter cupsugar gradually; add vanilla and beat. Spread jam on top of one layer cake. Place second layer cake on top. Cover sides and top of cake with whipped cream. Sprinkle with additional nuts, if desired.