01 - Preheat the oven to 375°F. Lightly grease a 9x13-inch baking dish or large casserole dish to prevent sticking.
02 - In a large mixing bowl, combine strawberries, blueberries, raspberries, granulated sugar, cornstarch, lemon juice, vanilla extract, and a pinch of salt. Gently toss until the fruit is evenly coated. Transfer the fruit mixture to the prepared baking dish and spread into an even layer.
03 - In a separate bowl, whisk together all-purpose flour, granulated sugar, baking powder, baking soda, and salt.
04 - Add cold, cubed unsalted butter to the flour mixture. Use a pastry blender or your fingertips to cut the butter in until the mixture resembles coarse crumbs.
05 - Stir in buttermilk and vanilla extract just until a soft, sticky dough forms, being careful not to overmix.
06 - Drop heaping spoonfuls of the biscuit dough evenly over the fruit mixture. Sprinkle the surface with coarse sugar if desired.
07 - Place the dish in the oven and bake for 38 to 42 minutes, or until the biscuit topping is golden brown and the fruit filling is bubbling enthusiastically.
08 - Remove from the oven and let rest for at least 15 minutes to set. Serve warm, ideally accompanied by vanilla ice cream or whipped cream.