In a large mixing bowl combine sugar, flour, and salt. Stir in melted butter, lemon peel, and lemon juice. In a small bowl combine egg yolks and milk; add to flour mixture. In a mixer bowl beat egg whites to stiff peaks. Gently fold into lemon batter. Turn into an ungreased 8x8x2 baking pan.Place in a larger pan on oven rack. Pour hot water into larger pan to a depth of 1". Bake in a 350 degree oven for 35-40 minutes until top is golden and springs back when touched. Serve warm or chilled in individual dessert dishes. Makes 6-8 servings.