Nonstick cooking spray
2 1/2 pounds ripe peaches, pitted and cut into large chunks
1 tablespoon cornstarch
1/2 cup plus 2 tablespoons sugar
3/4 cup all-purpose flour
2 tablespoons ground flax seed
1/4 teaspoon kosher salt
3 tablespoons cold unsalted butter, cut into small pieces
2 tablespoons reduced-fat buttermilk
Non-fat vanilla yogurt or frozen yogurt, for serving, optional