Heat oven to 180C/160C fan/gas 4.
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 whizz until the mixture is
pale. Add the citrus juice, egg yolks, flour
and milk, and whizz 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-ish underneath. Serve dusted with
icing sugar, if you like.