Begin by assembling all your ingredients. Preheat your oven to 350°F (175°C) and lightly grease a 9x13-inch baking dish.
Prepare the Berries: In a small bowl, whisk together the sugar and cornstarch until well combined. Place the fresh blackberries in a large mixing bowl, then drizzle them with the melted butter. Sprinkle the sugar-cornstarch mixture over the blackberries and toss gently to coat each berry evenly. Spread the coated blackberries into the prepared baking dish, ensuring they are evenly distributed.
Make the Batter: In another mixing bowl, whisk together the flour, sugar, baking powder, and salt until they are well blended. Gradually stir in the milk, melted butter, and vanilla extract until the mixture is combined. The batter should be mostly smooth but slightly lumpy.
Pour the batter over the blackberries in the baking dish, making sure it covers the berries evenly.
Place the dish in the preheated oven and bake for 55 to 60 minutes, or until the blackberries are tender and the crust has turned a golden brown.
Serve warm and enjoy!