Preheat the oven to 300°F and line a baking sheet with parchment paper. Draw a 9-inch circle on the parchment and flip it over.
In a clean, dry mixing bowl, beat egg whites until soft peaks form. Gradually add granulated sugar and continue beating until stiff peaks form and the mixture is glossy.
Sift cocoa powder over the egg whites, add white vinegar and pure vanilla extract, and fold gently until combined. Fold in the chopped bittersweet chocolate.
Mound the meringue onto the parchment within the circle, creating a slight well in the center. Bake for 1 hour 30 minutes, then turn off the oven and let cool completely inside the oven.
Once cooled, whip the heavy cream to soft peaks and spoon over the pavlova. Top with Fresh mixed berries before serving.
