In a large bowl, using an electric mixer, beat the yolks at medium speed until thick and pale, 3 minutes. Beat in 2/3 cup of the sugar; increase the speed to high and beat until thick, about 5 minutes. Beat in the vanilla. In a medium bowl, using clean beaters, beat the egg whites and salt at medium speed until soft peaks form. Gradually add the remaining 1/3 cup of sugar and beat at high speed until the whites are stiff and glossy. Fold the egg whites and ground walnuts into the yolk mixture in 3 batches; fold just until no streaks remain.