Gently rinse your blackberries and remove any stems or debris. If using frozen berries, thaw them completely and drain excess liquid to prevent a soggy cobbler. Place berries in a large mixing bowl and set aside.
In a small bowl, whisk together sugar, cornstarch, lemon juice, lemon zest, cinnamon, and salt until well combined. Pour this mixture over the blackberries and gently fold until all berries are coated.
Preheat your oven to 375°F (190°C). Lightly butter a 9x13 inch baking dish or a 10-inch cast-iron skillet.
In a large bowl, whisk together flour, granulated sugar, brown sugar, baking powder, and salt. Using your fingers or a pastry cutter, work the cold butter cubes into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
Create a well in the center of your dry ingredients and pour in the buttermilk and vanilla extract. Stir gently until just combined – overmixing will result in a tough topping. The batter should be thick and slightly sticky.
Transfer the berry mixture to your prepared baking dish, spreading it evenly. Drop spoonfuls of the cobbler topping over the berries, leaving some gaps for the filling to bubble up.
Sprinkle the optional coarse sugar over the top for extra crunch and sparkle. Bake for 45-50 minutes until the topping is golden brown and the filling is bubbling around the edges.
Allow the cobbler to cool for at least 20 minutes before serving. This resting period allows the filling to set properly.