Combine sugar, flour, salt, and butter; add lemon juice and peel. Add to combined egg yolks and milk; mix well. Fold in egg whites and our into greased custard cups. Bake in pan of hot water in moderate oven 325 degrees for 45 minutes. When baked, each dessert will have custard on the bottom and sponge cake on top. This may be baked in greased shallow baking dish. Serves 8.