Preheat the oven to 400 degrees. Wash and dry the blackberries. Set aside. Grease a 9x13 baking dish with butter, dust with flour and set aside.
In a small saucepan, bring 1/2 cup of water to a boil. In a large bowl, pour in the flour, 1/2 cup of sugar, 1/2 teaspoon cinnamon, baking powder, and salt. Using a fork or pastry blender cut in the cold butter until you have a crumb-like mixture. Pour the boiling water over the crumb mixture. Stir until the mixture is moist.
In a medium saucepan dissolve the cornstarch in the remaining 1/4 cup of water. Stir in remaining 3/4 cup sugar. Add the lemon juice, vanilla, remaining 1/2 teaspoon of cinnamon and fresh blackberries. Mix until well blended. Bring to a boil over medium-high heat, stirring frequently.
Pour the blackberry mixture in the prepared baking dish. Next, using a large spoon, drop the crumb mixture into the baking dish by spoonfuls. Place the baking dish into the preheated oven uncovered for 20-25 minutes, or until the tops of the cobbler are golden brown. Serve with whipped cream or ice cream. Enjoy!