Lemon Sponge Pudding with Lemon Sauce

Lemon Custard Sponge Pudding
This is between a cake and a pudding. A sponge cake layer rises and forms on the top, with a creamy lemon custard below. The sauce adds another very tart and lemony note.
Serve the Lemon Custard Sponge Pudding in bowls with the Lemon Sauce spooned over. I like it served warm, with warm sauce ladled on top, or it's also lovely chilled on a hot summer day.

2 T butter, softened
1/3 cup sugar
3 eggs, separated
zest and juice of 2 lemons (1/4 cup juice and 2 tsp zest)
pinch of salt
1 cup milk
4 T flour

Beat the egg whites until stiff but not dry, then set aside. In a separate bowl, beat the yolks with the soft butter until lemon colored, then add the sugar, lemon zest, and salt. Add the milk, lemon juice, and flour alternately. Fold in the egg whites. Butter or spray a small casserole dish and pour in the mixture. Put that in another ovenproof dish (I used a square glass dish), and pour hot water into the surrounding dish. Bake at 350° for one hour.

Lemon Sauce
Juice of 2 lemons (or 1 lemon and one lime)
2 tsp lemon zest
4 tsp cornstarch dissolved into 1 cup cool water
1/4 - 1/3 cup sugar (I like it pretty tart)
1 T. butter
Place all ingredients in a saucepan and stir over medium heat until it starts to boil. Turn the heat down and stir and cook for three - four minutes.