Preheat oven to 375°F (190°C). If using peaches, peel (optional) and slice peaches to yield about 6 cups. If using berries, measure 6 cups and rinse, pat dry.

In a large bowl combine the sliced fruit, 3 tablespoons granulated sugar, 2 tablespoons light brown sugar, 1 tablespoon lemon juice, 2 tablespoons cornstarch, 1 teaspoon vanilla extract, 1 teaspoon ground cinnamon, and 1 teaspoon salt. Toss gently until fruit is evenly coated and cornstarch is fully incorporated.

Let the mixture sit for 10 minutes to macerate and release juices. Transfer fruit and accumulated juices to a 9x13-inch baking dish (or similar capacity). Spread into an even layer.

Biscuit topping

In a medium bowl whisk together 2 cups all-purpose flour, 3 tablespoons granulated sugar, 1 tablespoon baking powder, and 1 teaspoon salt until evenly mixed.

Cut 6 tablespoons cold unsalted butter into small cubes and use a pastry cutter or two forks (or pulse briefly in a food processor) to cut butter into the dry ingredients until the mixture resembles coarse crumbs with pea-sized bits of butter.

Make a well in the center and pour in 1 cup cold buttermilk. Stir gently with a wooden spoon or rubber spatula until just combined — do not overmix; dough should be shaggy and slightly sticky. If dough is too dry, add up to 1 tablespoon additional buttermilk.

Drop the biscuit dough by heaping 1/4-cup scoops (or large spoonfuls) evenly over the fruit, leaving small gaps; the dough will spread as it bakes. Aim for about 12 drops across the dish.

In a small bowl beat 1 egg with 1 teaspoon water to make an egg wash. Brush the tops of the biscuits lightly with the egg wash and sprinkle 1 tablespoon coarse sugar over the tops if desired for sparkle and crunch.

Baking and finishing

Place the assembled cobbler in the preheated 375°F (190°C) oven on the middle rack. Bake until the topping is golden brown and the filling is bubbly, about 35–45 minutes. If the topping browns too quickly, tent loosely with foil for the last 10–15 minutes.

Remove from oven and let cool for at least 15 minutes to let the filling thicken. Serve warm, topped with vanilla ice cream or whipped cream if desired.