Preheat the oven to 350 degrees. In a mixer fitted with a whisk attachment (or using a hand mixer), 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 4 minutes more.
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). Place the egg whites into the circle, using a spatula to spread out meringue to make the disk.
Bake in the center of the oven for 10 minutes, then reduce the heat to 300 degrees and bake until the meringue has puffed up and cracked on the top and the surface is lightly browned to the color of café au lait, about 45 minutes more.
Whip the cream and brown sugar together until stiff, then drizzle in the rosewater and mix 15 more seconds. Spoon it in the center of the cooled pavlova and spread out to within ½ inch of the edge. Arrange the fruit over the cream. Place the rose pedals on and serve in wedges.
To make the crystallized rose petals whisk the egg white to break it up a bit. Dip the petal in to coat both sides, rubbing off the excess. Cover in granulated sugar on both sides, shaking off any excess. Place on a sheet pan to dry at room temperature overnight. They will be crisp in the morning and retain their color.