Begin by assembling a bain marie or water bath in which the cakes will bake. Place a clean dish cloth or towel (this keeps the ramekins from moving around in the bath) in a large roasting pan or baking dish. Make sure all 4 ramekins will fit side by side in the pan and squarely on top of the dish cloth FLAT. Boil water in a tea kettle and have ready.
Preheat your oven to 325°F. Butter or spray with cooking spray the bottoms and sides of the ramekins if desired. I am using new ramekins so elect not to butter, but if you will use vintage custard cups, bakeware in which foods tend to stick, or bakeware that does not have a glazed interior finish, then consider buttering or using nonstick spray.
Using an electric mixer with the whisk attachment, beat the room temperature egg whites in a medium bowl on high speed until stiff peaks form. Set aside. In a separate bowl or the bowl of a stand mixer, beat the room temperature egg yolks together with the sugar until grainy.
To the sugar mixture, add in the milk, flour, lemon juice, zest and salt beating until smooth. Gently fold half the egg whites into the lemon mixture until well incorporated followed by the remaining egg whites until just a few lumpy white streaks are visible.
Distribute the cake batter equally between the four prepared ramekins, setting each one down in what will become the hot water bath and atop a clean dish cloth or towel that can lie flat inside. Use a rubber spatula to scrape out all of the batter.
Transfer the pan to the oven rack (it helps to slide the rack out just enough for the kettle). Pour boiling water into the pan to a depth of 1-inch, careful not to pour water into any of the ramekins. Slide the rack back into the oven CAREFULLY.
Bake until the tops are golden brown and the cake has pulled away from the sides of the ramekins, about 45 minutes. Remove carefully (the bain marie makes the entire setup HEAVY and hot!) to clean counterspace and use a rubberized oven mitt or rubberized canning tongs to remove the little lemon pudding cakes from the hot water bath to a wire rack to cool.
Garnish with fresh berries, lemon slices, or a dusting of Confectioners' sugar.