Heat the oven to 350°F. Grease a 1½-quart baking dish.

Whisk the egg yolks, sugar, flour, and salt in a bowl.

Stir in the milk, lemon juice, zest, and butter.

Beat the egg whites until soft peaks form. Fold them gently into the lemon mixture.

Pour the batter into the dish. Set the dish in a larger pan and add hot water halfway up the sides.

Bake for forty-five minutes, until the top looks lightly golden. Serve warm or chilled.