preheat oven to combine180·c

lightly butter an oven proof dish

in a bowl, beat the butter, sugar and lemon zest until light and combined

add the egg yolks gradually beating well after each addition

add the flour and milk to make a smooth batter

stir in the lemon juice. by now the batter may look like it has separated

in a separate bowl, whisk the egg whites until firm peaks form

using a metal spoon, gently fold the egg whites into the batter

pour the batter into the oven proof dish

pour boiling water halfway into a separate roasting tin

place the oven proof dish with the batter into the roasting tin

bake for 40 to 50 minutes, until firm to touch

allow to cool before serving

can be served with whipped cream or ice cream