Pre heat oven to 350 degrees. Butter an 8 inch glass baking dish.
1. Place the egg yolks , lemon zest, lemon juice and melted butter into a large bowl, whisk together and set aside.
2. In another bowl place the sugar, flour and salt, stir to combine. Whisk half the flour mixture into the egg mixture, whisk in half the milk, then whisk in the remaining flour mixture,whisk in the last half of the milk. The batter should be smooth.
3. Whip the egg whites until soft peaks form. Gently fold the egg whites into the batter.
4. Pour the batter into a buttered 8 inch glass baking pan or dish. Place a tea towel in the bottom of a roasting pan. Place the 8 inch baking dish with the batter on top of the towel. This insures that your baking dish isn't going to be sliding around the pan when you pour the water into it.. Pour hot water into the roasting pan until it's almost half way up the side of the 8 inch pan with batter in it. Bake in a pre heated 350 degree oven for 45 minutes or until the middle of the pudding is set. Remove the 8 inch glass pan from the roaster to a cooling rack. Cool completely, serve. Refrigerate leftovers.