Add 1/4 c. water to cornstarch and mix with lemon juice, sugar and blackberries in pan on stove. Bring to boil.
Preheat oven at 400. Place cake pan on cookie sheet or foil. Mix flour, salt, baking powder, butter together into coarse mixture, then add 1/4 c. boiling water to form dough. Pour blackberries into 8 x 10 cake pan, top with spoonsful of the dough and bake 30 minutes or until brown. Serve warm with vanilla ice cream or cool whip topping.