Preheat an oven to 350 degrees F (180 degrees C). Butter an 8-inch (20 centimeter) square baking pan. Bring a kettle of water to a boil.

In a bowl, stir together the sugar, flour and 1 pinch of salt. In another bowl, whisk together the egg yolks, milk, lemon zest and lemon juice. Pour over the flour mixture and stir until smooth. In a third bowl, beat the egg whites with the remaining pinch of salt until they hold stiff peaks. Using a rubber spatula, gently fold the whites into the egg yolk mixture until just a few streaks of white remain.

Pour the batter into the prepared pan and set in a larger baking pan. Place in the oven and pour boiling water into the large pan until it comes about halfway up the sides of the smaller pan.

Bake the pudding until golden brown on top, about 40 minutes. Let cool slightly, then cut into squares and serve warm or at room temperature.