Melt and cool butter. Beat eggs, sugar, and salt until thick and pale. Add vanilla and lemon zest. In batches, sift flour over batter and carefully fold it in (to keep the air that you beat into the eggs.) Fold in the cooled butter. Refrigerate batter 1hour. Preheat oven to 425°F. Spoon batter into greased madeleine pan and bake 7min. They will look pale on top when you remove them. Unmold immediately.