Set 6 ramekins (6 oz. each) in a 9- by 13-in. baking pan. Set aside.
In a large bowl, whisk together egg yolks and sugar until thick and creamy. Stir in flour, milk, butter, lemon juice, and lemon zest.
In a large, clean bowl, whip egg whites and salt until firm (but not dry) peaks form. Stir one-quarter of beaten whites into egg yolk mixture until blended, then gently fold in remaining whites.
Pour batter into ramekins.
Pour enough hot water into the baking pan to come 1 in. up sides of the ramekins.
Bake until cake layers are set and tops are lightly golden, about 30 minutes.
Remove ramekins from water bath and let cool to room temperature before serving.
