Preheat oven to 350 degrees. Lightly butter a 2 quart shallow dish such an 9 inch square baking pan and set aside. Beat the butter until soft then gradually add the sugar, beating until well blended. Beat in the yolks one at a time, then add the milk, salt, flour, lemon zest and juice and beat to mix thoroughly. (Don't worry if the mixture looks curdled). Beat the egg whites with a pinch of salt until they form soft peaks. Then lighten the lemon egg mixture with 1/4 of the beaten egg whites, then fold the remaining egg whites into this lightened batter. Turn this mixture into the prepared baking pan and set it in a larger baking pan. Pour in enough boiling water to come half way up the sides of the baking dish with the pudding and bake for 50 minutes to an hour. The custard will separate into a soft custardlike mixture on the bottom and a cakier spongy like texture on the top. (If the top begins to brown after 35 minutes of baking loosely cover the top with buttered foil.) Serve warm or chilled with or without whipped cream.