Add the egg yolks, butter, confectioners' sugar, lemon zest, vanilla and salt to the mixing bowl and beat at moderate speed until creamy. Beat in one-fourth of the beaten whites. Transfer the batter to a large bowl. Fold in one–third of the flour, followed by one–third of the remaining whites; repeat with the remaining flour and whites until no streaks of white remain. Divide the batter between the prepared pans and spread evenly with an offset spatula. Bake the cakes for about 12 minutes, or until firm, shifting the pans halfway through baking.