Preheat oven to 350°F (175°C). Butter a 1.5-quart baking dish or 6 ramekins and place inside a deep roasting pan. Heat water until hot but not violently boiling, since extremely hot water can overcook the edges too quickly.

In a large bowl, beat egg yolks with sugar until lighter in color. Mix in softened butter, lemon zest, and fresh lemon juice. Add flour and salt, then slowly whisk in the milk until the batter becomes smooth and thin.

In a clean bowl, beat the egg whites until stiff peaks form. You should be able to lift the whisk and see the peaks stand tall without collapsing. This step creates the airy top layer, so take your time and don’t stop too early.

Fold the egg whites gently into the lemon batter in 3 additions. The mixture will look airy and a little foamy, which is exactly what you want. Pour into the prepared dish or ramekins.

Pour hot water into the roasting pan until it reaches halfway up the sides of the baking dish or ramekins. Bake 30–40 minutes (ramekins closer to 25–30) until the top is lightly golden and the center jiggles slightly like soft custard.

Let cool for 10–15 minutes before serving warm. For a firmer texture, refrigerate at least 2 hours before serving chilled. Dust with powdered sugar just before serving for the prettiest finish.