3. In a bowl, beat the egg yolks with the sugar. Stir in the butter and the lemon peel and juice. Fold in the flour.
4. In the bowl of an electric mixer, beat the egg whites until they are stiff but not dry. Fold the whites into the batter then spoon the mixture into the molds, filling them about 2/3 full. Bake the Madeleines for 20 minutes. Cool them in the pans for 2 minutes then remove them and cool completely, fluted side up, on a wire rack.