Preheat oven to 200C. Line a baking tray with aluminium foil.
In a large bowl, mix the flour, 1/2 cup sugar, baking powder and salt. Cut in butter until the mixture resembles coarse crumbs. Stir in 1/4 cup boiling water just until mixture is evenly moist.
In a separate bowl, dissolve the cornflour in cold water. Mix in remaining 1 cup sugar, lemon juice and blackberries. Transfer to a cast iron frypan and bring to a boil, stirring frequently. Drop dough into the frypan by spoonfuls. Place frypan on the foil lined baking tray.
Bake 25 minutes in the preheated oven, until dough is golden brown.