Grease an 8 inch square baking dish.
In a large saucepan, combine the blackberries, sugar, and cinnamon. Cook and stir until mixture comes to a boil.
Combine cornstarch and water until smooth; stir into fruit mixture. Bring to a boil; cook and stir for 2-5 minutes or until thickened.
Pour into prepared baking dish.
For topping, in a small bowl, combine the flour, sugar, baking powder and salt. Cut in butter using a pastry blender or two forks until mixture resembles coarse crumbs.
Stir in buttermilk just until moistened. Drop by tablespoonfuls onto hot berry mixture.
Bake, uncovered, at 350F for 30-35 minutes or until filling is bubbly and topping is golden brown. Serve warm, with whipped topping or ice cream if desired.