Preheat your oven to 250°F (120°C). Line a baking sheet with parchment paper and draw a 9-inch circle on it.
In a large bowl, beat the egg whites with an electric mixer on medium speed until soft peaks form. Gradually add the caster sugar, one tablespoon at a time, while continuing to beat on high speed until stiff peaks form.
Sprinkle the cornstarch, vinegar, and vanilla extract over the meringue mixture and gently fold them in.
Spoon the meringue mixture onto the prepared baking sheet inside the circle. Shape it into a wreath shape by building up the edges and creating a small well in the center.
Bake for 1 hour to 1 hour and 15 minutes, or until the meringue is firm and crisp on the outside but soft on the inside. Turn off the oven and let the pavlova cool completely in the oven with the door slightly ajar.
While the pavlova cools, whip the heavy cream with the powdered sugar and vanilla extract until soft peaks form.
Once the pavlova is cool, spread the whipped cream over the meringue base. Top with mixed berries and fresh mint leaves if desired.
Serve immediately or refrigerate for up to 4 hours before serving.