Preheat oven to 180 degrees C. Grease and flour a 20cm square pan.
In a large bowl combine sugar, salt and melted butter. Beat in the flour. Stir in the lemon zest and lemon juice. Combine milk and egg yolks and add to lemon mixture.
In a large glass or metal mixing bowl, beat egg whites until stiff peaks form. Fold 1/3 of the whites into the batter then quickly fold in remaining whites until no streaks remain. Pour batter into prepared pan.
Set cake pan in a water bath. Bake in the preheated oven for 40 minutes or until top is lightly browned. Serve warm or cold.