Preheat oven to 350 degrees.
Sift together 1 cup of sugar, flour, baking powder and salt. Set aside
Beat egg whites until stiff. Add the remaining 1/2 c. sugar gradually while continuing to beat egg whites. Set aside.
Without washing the beaters, beat the egg yolks until light yellow. Add the lemon rind, lemon juice, milk and butter. Gradually add flour mixture and beat until smooth.
Gently fold the stiff egg whites into the lemon mixture until no whites show. Pour into 2 qt. casserole. Place in pan of water and bake for 45 minutes. Chill for 1 hour before serving.