- Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a 9×13-inch baking dish.
- Prepare the berries: Whisk sugar and cornstarch together in a small bowl. Place blackberries into a mixing bowl and drizzle with melted butter. Sprinkle cornstarch mixture over top, then toss until evenly coated. Spread into the prepared baking dish.
- Make the batter: Whisk flour, sugar, baking powder, and salt together in a mixing bowl until evenly blended. Stir in milk, melted butter, and vanilla until combined but still slightly lumpy. Pour over berries in the baking dish.
- Bake in the preheated oven until berries are tender and the crust is golden brown, 55 to 60 minutes.
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