Heat the oven to 400°F with a rimmed sheet pan on the lower rack.
Toss the blackberries gently with the sugar, cornstarch, lemon juice and salt.
Fit one rolled dough disk into a 9-inch pie plate, add the filling, and dot with butter.
Cut the second rolled disk into strips, weave a lattice, trim and crimp the edge, and brush the pastry with egg wash.
Bake for 20 minutes, lower the oven to 375°F, and bake 30 to 35 minutes more until bubbles rise in the center gaps.
Cool for 3 hours so the berry filling can set.