Preheat the oven to 350°F (175°C). In a mixing bowl, combine blackberries, sugar, lemon juice, and cornstarch. Toss gently to coat the blackberries evenly.

In another bowl, mix together flour, sugar, baking powder, and salt. Stir in milk and melted butter until just combined.

Pour the blackberry mixture into a greased baking dish. Spoon the topping mixture over the blackberries, spreading it evenly.

Bake in the preheated oven for 30-35 minutes, or until the topping is golden brown and a toothpick inserted comes out clean.

Let cool slightly before serving. Enjoy warm, optionally with vanilla ice cream.