Preheat oven to 350 degrees F.
Grease a 9-inch square baking dish and set aside.
In a large saucepan, combine the blackberries, sugar, lemon juice, and lemon zest. Cook and stir until mixture comes to a boil.
Combine cornstarch and water in a small bowl until smooth and stir the slurry into the fruit mixture. Bring to a boil, cook, and stir for 2-5 minutes or until thickened.
Pour blackberry mixture into prepared baking dish.
In a medium bowl, combine the flour, sugar, baking powder and salt. Cut in butter using a pastry blender until mixture resembles coarse crumbs.
Stir in buttermilk just until moistened. Using a cookie scoop, drop spoonfuls on top of the hot berry mixture.
Bake uncovered for 30-35 minutes or until filling is bubbly and topping is golden brown.
Serve warm, topped with whipped topping or ice cream.
