Preheat your oven to 375°F (190°C). Butter a 9x13-inch baking dish.
In a spacious bowl, lightly mix the blackberries with sugar, cornstarch, lemon juice, and cinnamon until they are thoroughly coated. Pour into the prepared baking dish.
Whisk together flour, sugar, baking powder, and salt for the topping in a mixing bowl.
Incorporate the cold butter using a pastry cutter or a pair of forks until the blend looks like coarse crumbs. This technique is crucial for achieving that perfect, flaky texture that American bakers love.
Stir in buttermilk and vanilla extract just until combined. Don't overmix – a light hand yields tender results.
Spoon dollops of the dough onto the blackberry mixture. Don't worry about covering every spot; the dough will spread as it bakes.
Sprinkle with coarse sugar if desired.
Bake for 45-50 minutes until the topping achieves a golden brown color and the filling is bubbling around the edges.