Beat together egg yolks, lemon juice, lemon rind, and butter or margarine until thick and lemon colored. Combine sugar, flour, and salt; add alternately with milk to the yolk mixture, beating well after each addition.
Beat egg whites until stiff. Blend egg whites into batter on low speed of electric mixer. Pour into 8 inch square baking dish.
Place a pan of hot water in the oven, and set the baking dish into the pan. Bake at 350 degrees F (175 degrees C) for 45 minutes.