Preheat oven to 300°F (150°C). Line a large baking sheet with parchment paper and trace a 9-inch circle using a pencil and round pan as a guide, then flip the parchment pencil-side down.
Wipe the mixing bowl and whisk with a vinegar-dampened paper towel to remove any grease, then let dry. Separate 4 large eggs, placing only the whites in the bowl, and let them sit at room temperature for 20–30 minutes.
Beat the egg whites on medium speed until soft peaks form — about 2–3 minutes — so the peak curls gently when the whisk is lifted.
Increase speed to medium-high and add the 225g superfine sugar one tablespoon at a time, waiting 20–30 seconds between each addition. Continue whipping for 5–7 minutes until the meringue is thick, glossy, and smooth with no sugar grains when rubbed between fingers.
Stop the mixer and add the 1 tsp vinegar, 1 tsp cornstarch, and 1 tsp vanilla extract. Gently fold in with a rubber spatula using sweeping circular motions until just combined with no streaks.
Spoon the meringue onto the traced circle and spread with the back of a spoon, keeping the edges slightly higher than the center to create a well for the toppings.
Place the baking sheet in the oven and immediately reduce the temperature to 250°F (120°C). Bake for 1 hour and 15 minutes without opening the oven door, then turn the oven off and leave the meringue inside to cool for at least 1 hour (or overnight) with the door closed.
Whip the cold heavy cream, powdered sugar, and vanilla extract together on medium-high speed until soft, billowy peaks form, about 2–3 minutes. Use immediately or refrigerate, covered, for up to 30 minutes.
Transfer the cooled meringue to a serving platter, spoon the whipped cream into the center well, and scatter the fresh berries over the top. Garnish with fresh mint leaves and serve immediately.