Preheat the oven to 180 C, 350 F, Gas Mark 4.

Butter a 900ml (1 1/4 pint) deep baking dish.

Beat the butter and sugar together until pale.

Beat in the egg yolks, a little at a time.

Add the lemon juice and zest, and gently fold in the flour, adding the milk a little at a time.

Whisk the egg whites and fold in gently.

Pour the mixture into the baking dish and bake in the centre of the oven for 40-45 minutes.

lemon sauce can be served hot or cold.