Preheat oven to 250°F (120°C). Line a baking sheet with parchment paper. Draw 3-inch circles as guides if you want even sizes, then flip the paper over.
Whip egg whites in a clean, dry bowl until soft peaks form.
Add sugar slowly—1 tablespoon at a time—while whipping until the meringue is glossy and forms stiff peaks.
Stabilize mixture by folding in cornstarch, vinegar (or lemon juice), and vanilla.
Shape pavlovas by spooning meringue onto the parchment circles. Use the back of a spoon to make a shallow well in the center of each.
Bake low and slow for 60–70 minutes until dry and crisp on the outside.
Cool in oven with the door slightly open until completely cooled. Don’t rush this step.
Whip cream with powdered sugar and vanilla until soft peaks form.
Assemble pavlovas just before serving. Spoon whipped cream into the wells and top with fresh berries.