Preheat oven to 375 degrees F (190 degrees C). Butter and flour 12 (3 inch) madeleine molds; set aside.
Melt butter and let cool to room temperature.
In a small mixing bowl, beat eggs, vanilla and salt at high speed until light.
Beating constantly, gradually add sugar; and continue beating at high speed until mixture is thick and pale and ribbons form in bowl when beaters are lifted, 5 to 10 minutes.
Sift flour into egg mixture 1/3 at a time, gently folding after each addition.
Add lemon zest and pour melted butter around edge of batter. Quickly but gently fold butter into batter. Spoon batter into molds; it will mound slightly above tops.
Bake 14 to 17 minutes, or until cakes are golden and the tops spring back when gently pressed with your fingertip.
Use the tip of the knife to loosen madeleines from pan; invert onto rack. Immediately sprinkle warm cookies with granulated sugar. Madeleines are best eaten the day they're baked. Leftover madeleines are wonderful when dunked into coffee or tea.
Variation: Chocolate Madeleines: Omit lemon zest. Increase sugar to 1/2 cup. Substitute 1/4 cup unsweetened cocoa powder for 2 tablespoons of the flour; sift into batter with flour.