Preheat the oven to 180 C / Gas 4. Lightly grease a 20x30cm (9x13 in) baking tin.
Whisk 100g sugar with cornflour in a small bowl; set aside.
Place blackberries into a mixing bowl and drizzle with 60g melted butter. Sprinkle with cornflour mixture and toss evenly to coat. Spread blackberries into the prepared tin. In a separate bowl, whisk together flour, 300g sugar, baking powder and salt until evenly blended. Stir in milk, vanilla and 60g melted butter until combined but still slightly lumpy. Pour over berries.
Bake in the preheated oven until berries are tender and crust is golden brown, 55 to 60 minutes.