Line a baking sheet with aluminum
In a large bowl, mix the flour, ¾ cup sugar, baking powder, and salt.
Cut in butter until the mixture resembles coarse crumbs.
Stir in ¼ cup boiling water just until mixture is evenly moist.
In a separate bowl, dissolve the cornstarch in cold water.
Mix in remaining sugar, lemon juice, and blackberries.
Transfer to a cast iron skillet, and bring to a boil, stirring frequently.
Drop dough into the skillet by spoonfuls. Place skillet on the foil lined baking sheet.
Bake 25 minutes in the preheated oven, until dough is golden brown.
