01 - Preheat the oven to 375°F. Lightly grease a 2-quart 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 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 or clean fingertips, work the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized butter pieces remaining.
05 - In a small bowl, whisk together milk and egg until blended. Pour over the flour-butter mixture and stir with a spatula just until combined. Do not overmix; a few small dry patches are acceptable.
06 - Drop spoonfuls of the biscuit topping evenly over the fruit filling, leaving small gaps between mounds for steam to escape. Sprinkle with coarse sugar if desired.
07 - Bake for 38-42 minutes until the topping is golden brown and the fruit filling is bubbling around the edges. If the topping begins to brown too quickly, tent loosely with aluminum foil for the final 10 minutes of baking.
08 - Remove from oven and let cool for 15-20 minutes before serving. The filling will thicken slightly as it cools. Serve warm, topped with vanilla ice cream if desired.