Preheat the oven to 350 degrees. Combine the sugar, flour and salt in a large bowl. Stir in the melted butter, lemon peel and lemon juice. Combine the egg yolks and milk in a small bowl. Add to the flour mixture. Beat the egg whites using an electric mixer until stiff peaks form. Gently fold the egg whites into the lemon batter. Turn into an ungreased (8"x8"x2") baking pan. Place inside a larger pan on a rack in the oven. Pour hot water to a depth of 1" into the outer pan. Bake until the top of the cake is golden and springs back when touched (35-40 minutes). Serve warm or cold in individual dessert dishes.