1. Combine sugar, flour, and salt in a bowl; add a mixture of the beaten egg yolks, lemon peel and juice, and melted butter; mix well. Stir in the milk.
2. Beat egg whites until stiff, not dry, peaks are formed. Fold into first mixture. Pour into 6 custard cups.
3. Bake in a pan with hot water in a 350°F oven about 35 minutes, or until golden brown on top.