Preheat your oven to 275°F (135°C) and line a baking sheet with parchment paper.
In a large, clean, and dry bowl, beat the egg whites until soft peaks form. Gradually add the sugar, about a tablespoon at a time, and continue beating until the meringue is glossy and holds stiff peaks.
Sprinkle the sifted cocoa powder, cream of tartar, vanilla extract, vinegar, and cornstarch over the meringue. Fold everything in gently with a spatula until the cocoa is thoroughly mixed in.
Spoon the meringue onto the parchment paper, forming a circle about 9 inches in diameter. Use a spatula to shape the edges and make a slight well in the center to hold the toppings later.
Place the meringue in the oven and immediately reduce the temperature to 250°F (120°C). Bake for about 1 hour and 15 minutes, or until the pavlova is dry to the touch. Turn off the oven and let the pavlova cool completely inside with the door slightly ajar. This can take a couple of hours and helps prevent cracking. Once the pavlova is cool, carefully transfer it to a serving plate. Whip the tablespoon of cocoa powder and heavy cream with the 2 tablespoons of sugar until soft peaks form, and spoon it over the top of the pavlova.
Garnish with fresh berries and chocolate shavings or a dusting of cocoa powder just before serving.