Preheat the oven to 400 degrees F.

Roll half of the pie dough into a 12-inch circle. (Some homemade crust recipes recommend a 13-inch circle instead. Either works if it comfortably fits your pie pan.)

Transfer the bottom crust into your 9-inch pie plate. Gently press it against the edge of the pie dish.

In a large bowl, combine the fresh blackberries, sugar, cornstarch, and lemon juice. Stir very gently with a wooden spoon until berries are evenly coated.

Pour the berry mixture into the bottom crust and spread evenly.

Dot the top of the filling evenly with the butter cubes.

Roll the second crust and place it over the pie. Trim any excess dough around the edges. Crimp the edges to seal well. Cut a few small vents in the top crust.

Brush the top crust with the beaten egg wash. Sprinkle with sugar or turbinado sugar if desired.

Place the pie pan on a rack in the middle of the oven. On a lower rack, place a rimmed baking sheet lined with foil to catch any bubbling juices during baking.

Bake at 400 degrees for about 20 minutes, then reduce the oven to 375 degrees F and continue baking until the crust becomes golden brown, about 35-40 minutes. The filling should appear bubbly through the vents.

Shield the edge of the pie with aluminum foil if browning happens too quickly.

Allow the pie to cool completely on a wire rack. Cooling helps prevent a runny pie and allows the filling to set.