Preheat your oven to 375°F (190°C). In a large mixing bowl, combine the blackberries with 1 cup of sugar, cornstarch, and lemon juice. Gently stir until the berries are evenly coated and set aside, allowing the juice to macerate and thicken slightly.
4 cups fresh blackberries, 1 cup granulated sugar, 2 tablespoons cornstarch, 1 teaspoon lemon juice
Pour the berry mixture into a 9x13 inch baking dish, spreading it into an even layer. Place the dish in the oven and bake for 10-12 minutes, just until the filling begins to thicken and bubbles start to form around the edges.
Meanwhile, prepare the topping by combining flour, sugar, and salt in a bowl. Add the cold, small pieces of butter and cut it into the flour mixture using a pastry cutter or two forks until the mixture resembles coarse crumbs. Mix in warm milk and vanilla extract until just combined. Do not overmix.
1/2 cup all-purpose flour, 1/4 cup granulated sugar, 1/4 teaspoon salt, 1/4 cup unsalted butter, 1/4 cup milk, 1 teaspoon vanilla extract
Remove the partially baked berry filling from the oven and carefully drop spoonfuls of the topping mixture over the hot berry filling, covering most of the surface but leaving some gaps for steam to escape. Return the dish to the oven, baking for an additional 20-25 minutes, or until the topping is golden brown and the filling is bubbling vigorously.
Once baked, remove the cobbler from the oven and let it cool for about 10 minutes. The topping should be crisp and golden, and the filling thickened and bubbling. Serve warm, optionally with ice cream or whipped cream.
