Preheat oven to 350°F and line a baking sheet with parchment paper. Set aside.
In a large mixing bowl, use an electric mixer with a whisk attachment to whip the egg whites until foamy. Add in the cream of tartar and cornstarch and mix thoroughly.
Slowly add in the granulated sugar a little bit at a time, letting the mixer run in between additions. Use a spatula to scrape the sides of the bowl if any sugar gets stuck there.
Continue whipping the egg whites until it becomes stiff peaks - when you pull the whisk out and flip it upside down, the meringue tail should stand up on its own and gently bend over at the tip into a small hook shape.
Use a spatula to plop down six mounds of meringue onto your lined baking sheet. Use the spatula to gently shape the meringue into round shapes but don't worry too much. The organic look works well here.
Place the baking sheet into the oven and immediately turn the heat down to 250°F. Bake for 1 hour or until the surfaces of the pavlovas are dry and hard and the bottoms can peel cleanly off the parchment. Turn off the oven and keep the oven door closed. Leave the pavlovas inside to cool like that for at least 1 hour or overnight.
Meanwhile, in a small pan, bring blackberries, lemon juice, and water to a boil. Use a spatula to muddle the blackberries until it reduces down and thickens into a jammy consistency, about 5-7 minutes. If it's too thick and dry, you can add another tablespoon of water and stir thoroughly.
Push the blackberry sauce through a fine mesh sieve into a small bowl to remove the seeds. Scrape the outer bottom of the sieve to collect the liquid that doesn't drip down. Discard the seeds and excess pulp and set sauce aside.
In another mixing bowl, use an electric mixer or whisk to whip together whipping cream, powdered sugar, and vanilla extract until it thickens and becomes soft peaks.
When the pavlovas have cooled, use a spoon to crack open a hole on top of each pavlova to reveal an empty cavity inside. Spoon in whipped cream to fill the pavlova. Top whipped cream with a teaspoon of blackberry sauce and swirl with a chopstick or leave as is. Garnish with fresh blackberries.