Preheat oven to 180°C. Butter a 23cm round baking dish well.
In a bowl, whisk the egg yolks with the sugar until pale and slightly thickened.
Stir in the melted butter, lemon zest and juice.
Add the flour and mix until smooth.
Slowly whisk in the milk to create a thin batter.
In a separate bowl, whisk the egg whites with a pinch of salt to soft peaks.
Gently fold the egg whites into the batter. It should look loose and a little lumpy.
Pour into the buttered round dish.
Sit the dish in a roasting tray and fill halfway with hot water.
Bake for 35 to 45 minutes until lightly golden on top with a slight wobble underneath.
Serve warm. Spoon straight through the top to reveal the custardy, zingy layer underneath.
Serve with cream or ice cream.
