Preheat oven to 350 degrees F.
In a 2-quart mixing bowl combine, sugar, butter, flour, salt and milk, mixing until smooth. Add egg yolks to this and mix until smooth. Add lemon juice and zest.
In a mixer, beat egg whites until peaks are stiff. Fold the yolk mixture into whites gently to combine mixtures.
Using butter or vegetable shortening, grease 6-inch custard cups. Pour the mixture into the cups. Set the cups in 1/2 inch of hot water in a 9 by 12-inch baking pan. Bake 25 minutes or until tops have risen and turn golden brown.