Preheat oven to 350F. Set a spray-coated souffle dish or other straight-sided baking dish in a larger baking pan that is at least 2-inches deep; set aside.
In a deep bowl, beat egg whites with an electric mixer on high speed until foamy. Gradually beat in 1/4 cup of the sugar; continue to beat until whites hold short, distinct peaks. Set aside.
In another bowl, combine butter, remaining 1/2 cup sugar, lemon peel and egg yolks. Beat on high speed until mixture is thick and lemon-colored. Stir in buttermilk, lemon juice and flour. Add about 1/4 of the egg whites to batter; stir to mix well, Gently, but thoroughly, fold in remaining whites.
Pour batter into souffle dish. Set dish (in baking pan) on middle rack of oven. Pour boiling water into BAKING PAN up to the level of the batter in the souffle dish. Bake until top of pudding is a rich brown and center feels firm when lightly pressed, about 1 hour. Serve pudding hot or cool, spooning portions up from the bottom of the baking dish to get the lemon sauce that has formed beneath the pudding.