Preheat oven to 325 degrees F. Adjust oven rack to center position.
Lightly butter or spray with non-stick spray the individual custard cups, or ramekins of your choice. If you do not have individual custard cups, bake the batter in an 8-inch square baking dish (or other shallow 2-quart baking dish). Bring several quarts of water to boil for water bath.
In a large bowl, combine butter, sugar, and salt; beat until crumbly. Beat in egg yolks and flour, mixing until smooth. Slowly beat in lemon zest and juice; stir in milk.
In a large bowl, beat egg whites until stiff peaks form. Gently whisk whites into batter just until no large lumps remain. Immediately ladle batter into prepared baking pan or custard cups. Do not pour; otherwise, the first cups get all the froth and the later cups get all the batter.
Set a roasting pan onto oven rack. Lay folded dish towel onto bottom of roasting pan; set custard cups or baking pan on top of towel. Pour enough boiling water into roasting pan to come halfway up sides of baking pan or custard cups. The gradual, even heat of baking in a water bath insulates the pudding cakes from the direct heat of the oven, preventing over-baking at the edges before the centers are set.
Bake approximately 20 to 25 minutes (for ramekins or custard cups) or 30 to 35 minutes (for 2-quart baking dish) or until pudding cake center is set and springs back when gently touched. Remove roasting pan from oven and set on a wire rack. Let pan or cups continue to stand in water bath for 10 minutes.
Serve at room temperature or chilled.
When baked in ramekins or custard cups, pudding cakes can be inverted after baking and served upside down. I prefer them served chilled, but if you prefer them served warm, allow enough time to let the pudding cakes rest first, about 30 minutes after removing them from the oven, so the custard can set properly. Served cold, they have more intense flavor, and the two layers are more distinct. If desired, sprinkle the individual pudding cakes with powdered sugar just before serving.
Makes 6 servings.
Lemon Pudding Cake Recipe: https://whatscookingamerica.net/Cake/lemonpudcake.htm