Place a large roasting pan on a rack in the center of the oven. Fill the pan halfway with water. With the pan inside, heat the oven to 350 degrees F. Butter an 8-inch square or round baking dish.
In a large bowl, whisk together the egg yolks, lemon zest, lemon juice and butter.
In another bowl, stir together the sugar, flour and salt.
Whisk half the flour mixture into the egg yolks, then half the milk. Whisk in remaining flour mixture, then remaining milk.
Whip the egg whites until soft peaks form, then gently fold them into the batter.
Pour batter into the prepared baking dish. Carefully place the dish in the pan of water in the oven. Bake until the cake is set, about 45 minutes.
Notes
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