Preheat oven to 180 degrees C. Beat together egg yolks, lemon juice, lemon zest and butter until thick. Combine sugar, flour and salt; add alternately with milk to the yolk mixture, beating well after each addition.
Beat egg whites until stiff. Gently fold egg whites into milk mixture. Pour into 20cm square baking dish.
Place a pan of hot water in the oven, and set the baking dish into the pan. Bake for 45 minutes.