Beat egg yolks and 1/2 cup sugar in a bowl until thick and creamy. Add butter, milk, lemon zest, lemon juice and flour and mix until thoroughly combined. Beat egg whites with electric mixer until soft peaks form. Add the remaining 1/2 cup sugar gradually, beating until dissolved between additions. Fold into lemon mixture, 1/2 at a time. Pour into lightly greased oven-proof dish (6 cup capacity) or into 6 individual 1 cup baking dishes. Place in large baking dish with enough hot water to come half way up the side of the smaller baking dish(es). Bake in 350F (180 °C) oven for 50 to 60 minutes (about 30 to 40 minutes for individual dishes) until pudding is set and top is light golden brown. Serves 6.