Preheat oven to 400 degrees F
Line a baking sheet with aluminum foil.
Grease a 12-inch skillet with non-stick spray or vegetable oil.
In a large bowl, mix the flour, 1/2 cup sugar, baking powder, and salt. To this mixture, cut in butter until the mixture resembles coarse crumbs. *video above for this procedure.
Next, to this mixture, stir in 1/4 cup boiling water just until the mixture is evenly moist.
In a separate bowl, dissolve the 2 tablespoons cornstarch in 1/4 cup cold water.
To the cornstarch mixture, mix in remaining 1 cup sugar, lemon juice, and blackberries.
Transfer berry mixture to a cast-iron skillet and bring to a boil. Stir frequently.
When berries begin to thicken slightly, drop the dough into the skillet by spoonfuls.
Place skillet on the foil lined baking sheet.
Bake 25 to 30 minutes at 400 degrees until top is golden brown.
Serve warm with vanilla ice cream