Preheat the oven to 250°F (120°C). Line a baking sheet with parchment paper.

In a mixing bowl, beat the egg whites with an electric mixer on medium speed until soft peaks form.

Gradually add the granulated sugar, one tablespoon at a time, while continuing to beat until the mixture is glossy and stiff peaks form.

Gently fold in the vinegar and cornstarch.

Spoon the meringue mixture onto the prepared baking sheet, forming four nests.

Bake for 1.5 hours, then turn off the oven and let the meringues cool completely inside the oven.

In a clean mixing bowl, whip the chilled heavy cream with powdered sugar until soft peaks form.

Once the meringues are cool, carefully remove them from the parchment paper.

Top each meringue nest with whipped cream and fresh blackberries.

Garnish with additional blackberries if desired.