Preheat the oven to 180°C and butter a 1 litre ovenproof basin or serving dish.
In a food processor, cream butter with zest and sugar, then add egg yolks.
Add flour and milk alternately to make a smooth batter.
Scrape mixture from sides of processor bowl and blend in lemon juice.
Transfer to a clean bowl.
Whisk egg whites until creamy and firm and fold gently into batter.
Pour batter into the prepared dish.
Stand dish in a larger baking dish and pour in hot water to come halfway up the sides of baking dish.
Bake for approximately 45 minutes.
Serve warm out of the oven with cold cream or ice-cream and berries.
