The sweet and sour taste of spring, now in cobbler form!
Turn the oven up to 400 F and line a baking sheet with foil.
In a big bowl, mix the flour, 1/2 cup sugar, baking powder, and salt. Cut in the butter until the mixture becomes coarse and crumbly. Stir in the boiling water until mixture is evenly moistened.
In another bowl, dissolve the cornstarch in the cold water. Mix in your last cup of sugar, as well as the lemon juice and blackberries. Transfer that stuff to a cast iron skillet and get it boiling, making sure to stir frequently. Drop the dough from the first bowl into the skillet by spoonfuls. Place the skillet on the prepped baking sheet.
Bake that skillet for 25 minutes in the oven. When the dough is golden brown, you're done.