Step 1: Preheat your oven to 375°F (190°C). Lightly butter a 9x13 inch baking dish.
Step 2: In a large bowl, gently combine the fresh blackberries with 1/2 cup granulated sugar, 2 tablespoons all-purpose flour, 1 tablespoon lemon juice, and 1 teaspoon vanilla extract. Toss gently to coat the berries evenly.
Step 3: Pour the blackberry mixture into the prepared baking dish and spread evenly. Set aside while you prepare the topping.
Step 4: In a separate medium bowl, whisk together 1 1/2 cups all-purpose flour, 1/4 cup granulated sugar, 2 teaspoons baking powder, and 1/2 teaspoon salt.
Step 5: Cut 1/2 cup (1 stick) of very cold unsalted butter into small cubes. Add the cold butter to the dry ingredients. Using a pastry blender, two knives, or your fingertips, cut the butter into the flour mixture until it resembles coarse crumbs with some pea-sized pieces remaining.
Step 6: In a small bowl, whisk together 1/2 cup milk and 1 teaspoon vanilla extract. Add the milk mixture to the flour and butter mixture, stirring with a fork until just combined. Do not overmix.
Step 7: Drop spoonfuls of the biscuit dough evenly over the blackberry filling. You can flatten them slightly or leave them as rustic dollops.
Step 8: Sprinkle the top of the cobbler with 1 tablespoon of granulated sugar.
Step 9: Bake for 40-45 minutes, or until the topping is golden brown and the berry filling is bubbly around the edges. If the topping starts to brown too quickly, loosely cover with foil for the last 10-15 minutes.
Step 10: Remove from the oven and let cool for at least 15-20 minutes before serving. This allows the filling to set slightly.
Step 11: Serve warm, ideally with a scoop of vanilla bean ice cream or a dollop of fresh whipped cream. To store any leftovers, I highly recommend using durable Basics Glass Food Storage containers to keep your dessert fresh.