Heat oven to 180C. Grease a medium-sized oval baking dish (we used 22 x 18cm) with butter. Combine the butter, caster sugar and zest in a food processor and beat until the mixture is pale.
Add the citrus juice, egg yolks, flour and milk, and beat until well mixed – it may look a little curdled. Scrape into a bowl.
Boil the kettle.
In a large, clean bowl, whisk the egg whites until firm but not stiff, then gently fold into the zesty mixture.
Scrape into the baking dish and put in a roasting tin half-filled with hot water from the kettle. Bake for 35-40 mins until the top is lightly browned and set, but the pudding is still soft underneath.
Serve warm dusted with icing sugar and Kefalos Vanilla Ice Cream.
