Step 1

Preheat an oven to 350 degrees F (175 degrees C). Lightly oil a 9x13-inch baking dish. Whisk 1/2 cup sugar with the cornstarch in a small bowl; set aside.

Step 2

Place the blackberries into a mixing bowl, and drizzle with 1/4 cup of melted butter. Sprinkle with the cornstarch mixture, and toss to evenly coat. Spread the berries into the prepared baking dish. In a separate bowl, whisk together the flour, 1 1/2 cup sugar, baking powder, and salt until evenly blended. Stir in the milk, vanilla extract, and 1/4 cup melted butter until combined but still slightly lumpy. Pour the batter over the berries.

Step 3

Bake in the preheated oven until the berries are tender and the crust is golden brown, 55 minutes to an hour.