Preheat an oven to 180 degrees C. Lightly oil a 23x30cm baking tray. Whisk 1/2 cup sugar with the cornflour in a small bowl; set aside.
Place the blackberries into a mixing bowl and drizzle with 60g of melted butter. Sprinkle with the cornflour mixture and toss to evenly coat. Spread the berries into the prepared baking tray. 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 essence and 60g melted butter until combined but still slightly lumpy. Pour the batter over the berries.
Bake in the preheated oven until the berries are tender and the crust is golden brown, 55 minutes to an hour.