Pit and dice 1 large peach; reserve for shortcake batter. Pit and thinly slice remaining peaches and place in large bowl. Toss with 1/2 cup granulated sugar; let stand.
Heat oven to 400°F. Lightly coat 2 baking sheets with nonstick cooking spray.
Sift together flour, baking soda, cream of tartar, salt and 1/4 cup granulated sugar into large bowl.
Beat together sour cream and egg in medium-size bowl. Stir in reserved diced peach.
Stir sour cream mixture into flour mixture just until blended; knead about 6 times in bowl if necessary to blend. Drop batter by scant 1/2 cups in 12 mounds onto prepared baking sheets, spacing 2 to 3 inches apart. Sprinkle tops with remaining 2 tablespoons granulated sugar.
Bake in 400°F oven 12 to 15 minutes, until golden and wooden pick inserted in centers comes out clean. Cool on racks slightly.
Meanwhile, beat cream in bowl until soft peaks form. Beat in brown sugar and vanilla; beat until soft peaks form.
Split warm shortcakes in half. Spread lightly with softened butter if you wish.
To serve, place bottom of biscuit, buttered side up, on serving plate. Spoon on 1/3 cup fruit mixture; top with dollop of cream. Place biscuit, buttered side down, on top; dollop with more cream. Garnish with peach slice.