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 sugar; let stand.

Heat oven to 400@. Lightly coat 2 baking sheets with nonstick cooking spray.

Sift together flour, baking soda, cream of tartar, salt and 1/4 cup sugar into large bowl. Beat together sour cream and egg n medium bowl. Stir in reserved diced peach. Stir sour cream mixture into flour mixture just til blended; knead about 6 times if necessary to blend. Drop batter by scant 1/2 cups in 12 mounds onto prepared baking sheets, spacing 2 to 3" apart. Sprinkle tops with remaining sugar.

Bake 12 to 15 mins, til golden and tester comes out clean. Cool on racks slightly. Meanwhile, beat cream in bowl til soft peaks form. Beat in brown sugar and vanilla; beat til 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 1/3 cup fruit mixture; top with dollop of whipped cream. Place biscuit, buttered side down, on top; dollop with more cream. Garnish with peach slice.