Preheat oven to 215°F/101°C.
In a mixer bowl fitted with a whisk attachment, beat egg whites on low-medium speed until soft peak forms. With the mixture still on, gradually add sugar in 3 additions, beating for 30 seconds between, then increase to high speed and beat until stiff peaks form. The meringue at this time will be glossy, and the sugar is fully dissolved. Add the vanilla extract, the cream of tartar and cornstarch, either fold them in by hand or beat them on low speed until just combined. Gently fold in sifted cocoa powder and melted chocolate until combined.
Using two spoons, scoop a mound of meringue and palace on parchment paper. Use the back of one spoon to create an indentation in the center of each mound. Transfer baking sheets to the oven.
Bake for 60-75 minutes until the pavlova is firm and dry at the edges. Turn off the oven and, without opening the door, let pavlova cool completely in the oven. Transfer to a wire rack to cool completely. Meanwhile, making sauce.