Beat the butter, sugar, lemon rind & egg yolks together until light and fluffy. Gently stir in the flour, milk & lemon juice.
Put the egg whites in a clean bowl & beat until they form stiff peaks but are not dry.
Fold the 2 mixtures together & pour into a well-greased 3-4 cup capacity ovenproof dish. Bake at 160C for 30-35 mins in a water-bath until the top is golden & firm to the touch.