100ml lemon, lime or grapefruit juice, or a mixture
3 large eggs, separated
50g plain flour
icing sugar, for dusting (optional)
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.