Preheat your oven to 250°F (120°C). Line a baking sheet with parchment paper, creating a 9-inch circle on the paper as a guide.
In a clean and dry mixing bowl, whip the egg whites with an electric mixer on medium speed until foamy.
Gradually add the granulated sugar, one tablespoon at a time, while continuing to beat on high speed until the mixture forms stiff peaks and is glossy. This should take about 5-7 minutes.
Gently fold in the cornstarch, vinegar, and vanilla extract, being careful not to deflate the egg whites.
Spoon the meringue mixture onto the prepared parchment paper, spreading it to fit within the 9-inch circle, and create a slight indentation in the center for the cream and berries.
Bake in the preheated oven for about 1 hour and 15 minutes, or until the meringue is crisp on the outside and slightly soft on the inside. Turn off the oven and let the pavlova cool completely inside with the door slightly ajar.
Once cooled, in a separate bowl, whip the heavy cream with the powdered sugar until soft peaks form.
Carefully transfer the meringue base to a serving platter. Top with the whipped cream, spreading it evenly into the indent.
Generously pile the mixed berries on top of the whipped cream, arranging them as desired.
Garnish with fresh mint leaves to add a pop of color and flavor.
Notes
Ensure the mixing bowl is clean and dry for best results.