In a large bowl, whisk together the flour, salt, baking powder, baking soda, sugar, and cinnamon until evenly mixed.
Add the cold butter and using two forks or a pastry cutter, cut the butter into the flour, coating each piece with flour until the butter is about pea-sized.
Then add the buttermilk and fold gently until a shaggy dough forms. Do not overmix. Cover and place in the refrigerator until ready for use.
Place your oven rack in the middle position and preheat to 375°F.
In a 10-inch cast iron skillet, add half of the blackberries, sugar, salt, lemon zest and juice and begin to cook over medium heat.
Meanwhile, in a small bowl stir together the cornstarch and water to create a slurry. Once the berries begin to soften and release their juices, stir in the cornstarch slurry. Let the mixture come to a vigorous simmer for about 2 minutes.
Remove the filling from the heat and stir in the remaining blackberries.
Remove the biscuit dough from the refrigerator. Using a greased 1/3 measuring cup, scoop out portions of the dough and place them evenly across the top of the blackberries. Press on the biscuits gently to spread them a bit. Brush the melted butter on top of the biscuits.
Place the skillet in the oven and bake until the filling is thickened and bubbling and the biscuits are golden brown, about 35-38 minutes.
Remove from the oven and let sit for 15 minutes, then serve and enjoy!