Preheat oven to 180°C and butter a 1 litre ovenproof basin or serving dish. Zest 1 of the lemons and juice both.
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 basin.
Whisk egg whites until creamy and firm and fold gently into batter. Pour batter into prepared basin. Stand basin in a baking dish and pour in hot water to come halfway up sides of basin. Bake for 1 hour.
Allow to cool a little before serving. I like lemon delicious best with pouring cream.
