Preheat & prep: Preheat oven to 250°F. Line a baking sheet with parchment. Trace 6-8 3-inch circles as guides and flip the paper.
Make meringue: In a clean bowl, beat egg whites on medium until foamy. Add cream of tartar. Gradually add sugar, 1 tbsp at a time. Beat until glossy, stiff peaks form. Fold in vanilla and cornstarch gently.
Pipe & shape: Spoon or pipe meringue into your circles, building up the edges like little nests.
Bake: Bake for 1 hour. Turn off oven, crack door, and let cool inside for another hour.
Whip cream: Beat cream, powdered sugar, and vanilla to soft peaks.
Assemble: Spoon cream into cooled pavlovas. Top with berries and mint. Serve immediately.