To make pastry, sift flour and salt into a large bowl.
Using a pastry blender or 2 knives, cut in butter until mixture resembles breadcrumbs.
With a fork, mix in sugar, egg and enough water to make a dough.
Wrap in foil and refrigerate 30 minutes.
To make filling, sprinkle blackberries with sugar and drain over a saucepan.
Warm berry juice.
Combine cornstarch and water; stir into berry juice.
Bring to a boil, stirring constantly.
Reduce heat and simmer; stir until thickened.
Add blackberries; cool.
Preheat oven to 325°F (165°C).
On a floured surface, roll out dough to fit an 8-inch (20-cm) flan tin with a removable bottom.
Place:dough in tin.
Bake blind,15 minutes or until golden; cool.
Raise oven temperature to 350°F (175°C).
Spread jam over pastry shell.
Add blackberry mixture.
To make meringue, beat egg whites until stiff.
Beat in powdered sugar.
Place in a pastry bag fitted with a fluted nozzle.
Pipe a lattice over blackberry filling.
Bake 15 minutes at 350°F (175°C).
Cool slightly in tin.
Cool completely on a rack.