Heat the oven to 175 C / 350 F. Lightly butter a 1.5-quart ceramic baking dish and set it inside a larger roasting pan.

In a large mixing bowl, whisk together the granulated sugar, sifted flour, and salt until combined.

Add the melted butter, lemon zest, lemon juice, egg yolks, and milk to the flour mixture. Whisk until smooth with no dry pockets.

In a separate clean bowl, beat the egg whites with an electric hand mixer on medium-high speed until stiff peaks form - the whites should hold their shape when you lift the beaters.

Scoop about one-third of the beaten whites into the lemon batter and stir gently to loosen it. Add the remaining whites and fold with a rubber spatula in slow, wide strokes until just combined. A few white streaks are fine.

Pour the batter into the prepared baking dish. Place the roasting pan with the dish into the oven, then carefully pour hot water into the roasting pan until it reaches halfway up the sides of the baking dish.

Bake for 33 to 35 minutes, until the top is set and lightly golden and the center has a slight jiggle when gently shaken.

Carefully remove the baking dish from the water bath. Let it cool for 5 minutes before serving. Dust with powdered sugar if using and spoon into shallow bowls, making sure each portion gets both the sponge top and the custard underneath.