Preheat oven to 180C.

Use six 200ml ramekins or a one-litre ovenproof dish. Grease ramekins or dish with butter, place in a large, deep baking pan and set aside.

Cream butter and sugar until light and fluffy, then add egg yolks. Add flour and milk alternately to make a smooth batter.

In a separate bowl, whisk egg whites until stiff peaks form. Fold into batter.

Stir in combined lemon and lime juice and zest.

Divide mixture into ramekins or dish and pour hot water into baking pan to halfway up dishes.

Bake in oven until set and browned: about 30 minutes for ramekins and up to one hour for a large dish.

Carefully remove from water bath before serving.