Beat the egg whites, salt, and cream of tartar starting on low and gradually increasing the speed to high until the egg whites start to get foamy. Gradually add the sugar, 1 tablespoon at a time, and beat until stiff, glossy peaks form. Beat in 1 teaspoon vanilla and the cornstarch.
Place 4 equally sized mounds of the meringue on the prepared baking sheet then, using the back of a spoon, form into rounds about 3 inches in diameter and create a little well in the center of each mound. Place in the oven, then immediately lower the heat to 200 degrees. Bake for 90 minutes or until firm and dry. Let cool completely.
Pit and slice the peaches and place in a bowl with 2 teaspoons of powdered sugar, gently toss to coat, and let sit at room temperature.
When the pavlovas are cool, whip the cream with the remaining 1 teaspoon vanilla and 2 teaspoons powdered sugar until stiff peaks form.
Spoon the whipped cream into the pavlovas, top with the peaches, pistachios, and mint leaves, and serve.