Preheat oven to 180° C (350° F). Lightly grease an 18 cm ovenproof dish .
Cream the butter, sugar and lemon zest until light and fluffy. Add the flour and beat until just incorporated. Then add egg yolks, milk and lemon juice and mix thoroughly.
In a fat free bowl, beat egg whites until stiff peaks and fold into the creamed mixture.
Pour the mixture into the ovenproof dish and place this dish into a roasting tin.
Pour enough water into the roasting tin at the base until the base is covered by approx. 2cm of water.
Bake in the oven until top is golden and springy to the touch.