Preheat oven to 250°F (120°C). Line a baking sheet with parchment paper and draw a 9-inch circle as a guide.
In a clean, dry bowl, beat egg whites with an electric mixer on medium speed until soft peaks form. Gradually add superfine sugar, 1 tablespoon at a time, beating on high until stiff, glossy peaks form. Beat in cornstarch, vinegar, and 1/2 tsp vanilla.
Spoon meringue onto the parchment, spreading to fill the circle. Create a slight well in the center. Bake for 60 minutes, then turn off oven and let cool completely inside with door slightly ajar.
Meanwhile, in a chilled bowl, beat cream, powdered sugar, and 1 tsp vanilla until soft peaks form. Refrigerate.
In a small saucepan, heat berry jam and water over low heat until melted. Let cool slightly.
To assemble, place meringue on a serving plate. Top with whipped cream. Arrange mixed berries on top. Brush berries with warm jam glaze. Serve immediately.
Tips
Ensure egg whites are at room temperature and free of any yolk for best volume.
Pavlova is best assembled just before serving to keep the meringue crisp.
Store leftover pavlova components separately: meringue in an airtight container, cream and berries refrigerated.