Preheat the oven to 220°F (100°C).
Separate the egg whites from the yolks carefully, ensuring no yolk gets into the whites.
In a completely clean mixing bowl fitted with a whisk attachment, add the egg whites and a splash of lemon juice. Whisk until frothy.
Add vanilla extract. Gradually add the sugar one tablespoon at a time while whisking continuously.
Scrape down the sides of the bowl occasionally to ensure all sugar dissolves properly.
Continue whisking until stiff peaks form. The meringue should look glossy and hold its shape firmly.
Line a baking sheet with parchment paper. Dab a little meringue underneath each corner of the parchment to keep it in place.
Spoon the meringue onto the center of the baking sheet and shape it into an 8–9 inch circle. Build the edges slightly higher to create a well in the center.
Use a palette knife or spoon to create decorative swirls around the edges if desired.
Bake on the center rack for 90 minutes until the outside feels crisp.
Turn off the oven and allow the pavlova to cool inside the closed oven for at least 1–2 hours.
Crack the oven door open slightly and let it cool for another 30–60 minutes.
Wash and prepare the berries. Add them to a bowl with sugar and lemon juice. Toss gently and let sit.
To make whipped cream, whisk the heavy cream with optional sugar and vanilla until soft peaks form.
Transfer the cooled pavlova to a serving plate. Spoon the whipped cream into the center well.
Top with the berry mixture and garnish with mint leaves if desired.
Serve immediately for the best texture.