In a mixer fitted with a whisk attachment, whip the egg whites, cream of tartar, and salt in a clean, dry bowl until foamy. Add the granulated sugar, cornstarch, vanilla, and vinegar and continue whipping until stiff, smooth, and glossy, about 5 minutes more. Add the sifted cocoa powder and mix 10 to 20 more seconds.
On a sheet of parchment paper cut to fit a sheet pan, use a pencil to draw or trace a circle 9-inches in diameter. Line the sheet pan with the parchment, pencil side down (you should still be able to see the circle). Spoon the egg whites into the circle, using the back of the spoon to smooth the top and sides of the disk.
Bake in the center of the oven for 10 minutes, then reduce the heat to 300 degrees F and bake until the meringue has puffed up and cracked on the top, about 45 minutes more. Turn off the oven, prop the oven door open, and let the pavlova cool in the oven at least 30 minutes, to room temperature. This ensures a gradual cooling, which protects the delicate meringue.
Before you are ready to serve, place the meringue on a serving platter and scoop the sorbet over the top of the meringue. Then put it back into the freezer until ready to serve. When ready to serve, arrange the fruit decoratively. To serve, slice into wedges with a serrated knife.