Put oven rack in middle position and preheat oven to 425º. Butter a 13x9x2-inch glass or ceramic baking dish.
Whisk together cornstarch, cinnamon, sugar, and peaches in a large bowl and toss to combine. Add the more delicate cherries last and give a gentle stir.
Transfer fruit mixture into the baking dish and bake for ten to fifteen minutes until bubbling. While fruit bakes, whisk the flour, baking powder, and salt together. Add buttermilk and melted butter (allow it to cool slightly before adding.) Mix just until a dough forms.
Drop batter onto fruit and sprinkle with remaining tablespoon of sugar.
Bake until top is golden brown, about 20 - 25 minutes.