Roll pastry to 21" X 14" To make this easy, get a poster and cut a couple of   1/4" strips longways then measure and cut 2 - 25" strips (from those). Wrap with a couple of layers of foil. Then form a frame that measures 21" X 14". Use more foil to hold the 2 pieces together. Then roll the pastry dough inside the frame. Then cut the pastry into 6 - 7" squares. Place a tablespoon of sugar in the center of each pastry. Then place about 1/4 to 1/3-cup blackberries on top of that sugar and sprinkle with a little more sugar.

Moisten the edges of the pastry a little and fold corners up and pinch together. Then pinch the sides together.
Preheat oven to 375 degrees. Bake for about 40 minutes or until golden.Or bake in a convection oven preheated to 325 degrees for about 25 to 30 minutes or until golden.

Meanwhile, time to make the sauce. Place 1 tablespoon sugar and the cornstarch in a small saucepan. Heat and stir in about 1 1/2-cup blackberries.Continue heating until it becomes thickened and bubbly. Press through a sieve to remove the seeds. Keep warm. In a small skillet place in 2 tablespoons of brandy and ignite then pour over the sauce. As soon as the flame dies spoon the sauce over the dumplings and serve immediately.

Note: You may want to omit the brandy step. However you may wish to allow the dumplings to cool as you make the sauce. That way no one will get burnt.