Preheat the oven to 180 degrees C. Butter a baking dish.
In a food processor combine the butter, sugar and lemon zest then process until smooth and creamy. Add in turn the lemon juice, egg yolk, flour and milk. Combine to a smooth batter.
Whisk the egg whites until firm but not stiff then fold the two mixtures together.
Pour the batter into the prepared baking dish then place it in a tray half filled with water. Place in the preheated oven and cook for 45 minutes to until the top is lightly browned. Dust with icing sugar and serve hot.