Preheat the oven to 350°F and set a rack in the middle position. Spray six (6-oz) ramekins with nonstick cooking spray.

In a large bowl, whisk together the egg yolks, milk, lemon zest, lemon juice, butter, and salt. Add the flour and sugar and whisk until smooth.

In the bowl of an electric mixer, beat the egg whites until soft peaks form. Spoon about ¼ of the egg whites into the lemon mixture and whisk until smooth. Add the remaining egg whites and, using a rubber spatula, gently fold into the lemon mixture until smooth.

Place the prepared ramekins into a 9-by-13-inch baking dish. Ladle the batter evenly into the ramekins and pour room temperature water into the pan to reach halfway up the sides of the ramekins. Bake for 45 to 50 minutes, until the cakes are puffy and lightly golden.

Carefully remove the ramekins from the baking dish and let cool on a rack for about 20 minutes before serving. Dust with Confectioners' sugar and serve with berries, if desired.

Make Ahead: The cakes can be made a day ahead of time. Once completely cool, cover the cakes with plastic wrap and store at room temperature. Before serving, preheat the oven to 350°F and heat for 10-15 minutes until warmed through.