Preheat oven to 300°F (150°C). Line a large baking sheet with parchment paper and draw an 8-inch circle as a guide.

In a large, clean mixing bowl, beat the egg whites with an electric mixer on medium speed until soft peaks form, about 2-3 minutes.

Gradually add the granulated sugar one tablespoon at a time while beating continuously on high speed. Continue beating until the meringue is thick, glossy, and holds stiff peaks when you lift the beaters, about 6-8 minutes total.

Gently fold in the vinegar and cornstarch with a spatula until just combined, being careful not to deflate the meringue.

Spoon the meringue onto the prepared circle, spreading it into a round disc with slightly raised edges to create a nest shape. Use the back of a spoon to create swirls and peaks.

Place in the oven and immediately reduce temperature to 250°F (120°C). Bake for 1 hour and 15 minutes until the pavlova is crisp on the outside and sounds hollow when tapped.

Turn off the oven without opening the door and let the pavlova cool completely inside for at least 1 hour, then crack the door open slightly for another 30 minutes.

When ready to serve, whip the cold heavy cream with powdered sugar and vanilla until soft peaks form. Spread the whipped cream over the cooled meringue base.

Top generously with fresh mixed berries, arranging them artfully over the cream. Slice and serve immediately while the base is still crisp.