Step 1: Preheat your oven to 250°F (120°C) and line a large baking sheet with parchment paper.
Step 2: In a very clean, dry mixing bowl (preferably a stand mixer), beat the egg whites on medium speed until soft peaks form.
Step 3: Gradually add the granulated sugar, 1 tablespoon at a time, continuing to beat on medium-high speed. Beat until the meringue is glossy, stiff, and holds firm peaks, and all sugar has dissolved.
Step 4: Gently fold in the white vinegar (or lemon juice) and cornstarch until just combined.
Step 5: Spoon or pipe 12 individual meringue nests (about 3-4 inches in diameter) onto the prepared baking sheet, creating a slight well in the center of each.
Step 6: Bake for 60-75 minutes, or until the pavlovas are firm to the touch and easily lift from the parchment. They should be white or very lightly golden.
Step 7: Turn off the oven and leave the pavlovas inside with the oven door slightly ajar for at least 1-2 hours (or overnight) to cool completely. This prevents cracking.
Step 8: Just before serving, combine the chilled heavy cream, powdered sugar (if using), and vanilla extract (if using) in a cold bowl. Beat until soft peaks form.
Step 9: Carefully transfer the cooled pavlova shells to a serving platter. Fill each well with a generous dollop of whipped cream and top with fresh assorted berries and any other desired garnishes.