01 - Preheat the oven to 375°F (190°C). Lightly grease a 2-quart (2-liter) baking dish with butter or cooking spray.
02 - In a large bowl, combine sliced peaches, raspberries, ½ cup sugar, cornstarch, lemon juice, and vanilla extract. Gently toss until fruit is evenly coated. Transfer the mixture to the prepared baking dish and spread into an even layer.
03 - In a separate medium bowl, whisk together flour, ¼ cup sugar, baking powder, and salt until well combined.
04 - Add cold, cubed butter to the flour mixture. Using a pastry cutter, two forks, or your fingers, work the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
05 - In a small bowl, whisk together milk and egg until blended. Pour over the flour-butter mixture and stir gently with a spatula just until combined—do not overmix as this will toughen the biscuits.
06 - Drop spoonfuls of the biscuit topping evenly over the fruit surface, leaving some spaces for steam to escape. If desired, sprinkle coarse or turbinado sugar over the topping for extra crunch and sweetness.
07 - Bake for 38–42 minutes until the biscuit topping is deep golden brown and the fruit filling is bubbling vigorously around the edges. If the topping begins to brown too quickly, loosely cover with aluminum foil for the final 10 minutes of baking.
08 - Remove from oven and let the cobbler rest for 15–20 minutes before serving. This allows the fruit filling to set slightly. Serve warm, topped with vanilla ice cream or whipped cream if desired.