01 - Preheat the oven to 375°F. Lightly grease a 2-quart baking dish with butter or non-stick spray.
02 - In a large mixing bowl, combine the peeled and sliced peaches, raspberries, granulated sugar, cornstarch, lemon juice, vanilla extract, and a pinch of salt. Toss gently until the fruit is evenly coated.
03 - Pour the fruit mixture into the prepared baking dish and spread it out in an even layer.
04 - In a medium bowl, whisk together the all-purpose flour, granulated sugar, baking powder, and salt until well combined.
05 - Add the cold cubed butter to the flour mixture. Using a pastry cutter or your fingertips, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
06 - In a small bowl, whisk together the whole milk and egg. Pour the wet mixture into the flour-butter mixture and stir gently just until combined. Be careful not to overmix, as this will make the topping tough.
07 - Drop spoonfuls of the biscuit dough evenly over the fruit filling, leaving small gaps between mounds to allow steam to escape during baking.
08 - Place the baking dish in the preheated oven and bake for 40 minutes, or until the biscuit topping is golden brown and the fruit filling is bubbly around the edges.
09 - Remove from the oven and let the cobbler cool for at least 15 minutes before serving. Serve warm with a scoop of vanilla ice cream or a dollop of whipped cream if desired.