Preheat oven to 160°C (325°F).
Prepare a medium-sized casserole dish by spraying with cooking spray.
Cream butter, sugar, eggs yolk and vanilla. Stir in flour, milk, lemon juice, orange juice and lemon and orange rind.
Place egg whites into a clean, dry bowl. Beat until stiff. Lightly fold into lemon mixture.
Pour into prepared dish.
Place into a moderate oven and bake for approx. 35 minutes or until cooked when tested.