Heat the oven to 120 °C / 250 °F, then cover a large baking tray with foil and lightly grease it. The prepared surface should look evenly coated so the delicate meringue can release more easily.

Whip the egg whites in a large mixing bowl until stiff peaks form, then add the superfine sweetener one tablespoon at a time before mixing in the cornstarch, vinegar, and vanilla extract. The finished meringue should be thick, glossy, and firm enough to hold a pronounced peak.

Spoon the meringue onto the prepared tray and gently gather it into a round mound. Keep the shape compact and even so the edges bake at a similar pace.

Bake for 90 minutes, until the sides turn lightly golden and the center remains tender, then switch off the oven and cool the pavlova completely inside with the door slightly ajar. The outer crust should dry without becoming deeply browned, and slow cooling helps limit sudden collapse.

After the shell has cooled, whip the heavy cream with the powdered sweetener until thick and smooth, spread it over the pavlova, add the berries, and refrigerate for at least an hour to set. The cream should form soft, spreadable swirls that hold the berries without weighing down the shell.