Position oven rack to the lowest setting, and remove middle rack. Preheat oven to 425°.
Lightly coat 6 (8-ounce) soufflé dishes with cooking spray. Sprinkle evenly with 2 tablespoons granulated sugar. Set aside.
Place peaches and 1/3 cup granulated sugar in a food processor; process until smooth. Combine peach mixture, cornstarch, juice, salt, and egg yolks in a medium saucepan, stirring well with a whisk; bring to a boil. Cook for 1 minute, stirring constantly with a whisk. Remove from heat; stir in butter. Cool 5 minutes.
Place cream of tartar and egg whites in a large mixing bowl, and beat with a mixer at high speed until soft peaks form. Add the remaining 1/3 cup granulated sugar, 2 tablespoons at a time, beating until stiff peaks form (do not overbeat). Gently stir 1/4 of egg whites into peach mixture, and gently fold in remaining egg white mixture. Gently spoon the mixture into prepared dishes. Sharply tap dishes 2 or 3 times on counter to level. Place dishes on a baking sheet, and place baking sheet on the bottom rack of 425° oven. Immediately reduce oven temperature to 350° (do not remove soufflés from oven). Bake for 28 minutes at 350° or until a wooden pick inserted in the side of soufflé comes out clean. Sprinkle evenly with powdered sugar. Serve immediately.