Preheat the oven to 375°F (190°C). Grease a 9-inch baking dish.
In a large bowl, toss together blackberries, 1/2 cup sugar, 1/4 cup flour, lemon juice, cinnamon, and 1/4 teaspoon salt. Pour into the prepared baking dish.
In another bowl, whisk together 1 cup flour, 1/3 cup sugar, baking powder, and 1/4 teaspoon salt. In a separate bowl, mix milk, melted butter, egg, and vanilla. Add wet ingredients to dry ingredients and stir until just combined.
Drop spoonfuls of batter over the blackberries. Do not spread; the batter will spread during baking.
Bake for 40–45 minutes, or until the top is golden brown and the filling is bubbly. Let cool slightly before serving.
Notes
Serve warm with vanilla ice cream or whipped cream for an extra indulgent treat.