01 - Preheat oven to 375°F. Lightly grease a 2-quart baking dish with butter or nonstick 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 all the fruit is evenly coated.
03 - Pour the fruit mixture into the prepared baking dish and spread it out into 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 in 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 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 surface of the fruit, leaving small gaps between mounds to allow steam to escape during baking.
08 - Bake on the center rack for 40 minutes, or until the biscuit topping is deep golden brown and the fruit filling is bubbling around the edges.
09 - Allow the cobbler to cool for at least 15 minutes before serving. Serve warm, optionally topped with vanilla ice cream or whipped cream.