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.
