Over the years, pudding cakes have had different names such as sponge pudding, cake top pudding, sponge custard, and upside down pudding. But no matter what the name, this informal dessert is basically pudding topped by a cake layer that forms during baking. They have remained popular because they are so easy to make.
In a large mixing bowl stir together sugar, and flour. Then stir in lemon peel, and melted butter. Stir together egg yolks and milk; add to the flour mixture. Stir just until combined.In a bowl beat egg whites with an electric mixer on medium to high speed until stiff peaks form (tips stand straight). Gently fold egg whites into the lemon batter. Spoon into an ungreased 1 quart casserole dish.Place casseraole dish in a 13 x 9 x 2 inch baking pan set on the oven rack. Pour boiling or (hottest tap water) into the baking pan to a depth of 1 inch. Bake in a 350F oven for 40 to 45 minutes or until done. Serve warm with whipped cream, if desired.