Preheat oven to 400°F, place the rack on the lower third of the oven.

In a large bowl, pour the blackberries in and add the sugar, cornstarch, lemon juice, and vanilla. Stir until well combined. Set aside.

In a food processor with a steel blade, place the flour , sugar, and salt for the dough and pulse a few times to combine. 

Add butter cubes and pulse 10-12 times until butter pieces are the size of small peas. Add cold water and pulse again until the dough comes together.

Remove from food processor and place onto a lightly floured surface. Roll into a ball, then cut in half. Form each half into a round disk, place one disk into the refrigerator. 

Use a rolling pin to roll the remaining disk into a circle about 12" in diameter, turning a quater turn between rolls. Place dough into a 9" pie dish that is 2" deep. Smooth pie crust in the dish but do not press it down.

Stir the blackberry mix again and transfer fillng to the pie crust. Set aside.

Roll out the second pie dough disk in the same manner. You can create a lattice top if you like, or place the entire crust on top of the filling and cut slits in the center of the pie to allow steam to escape. 

Trim the edges of the pie crust so there is only 1/2" overhang, fold the excess 1/2" of dough under and use a fork to crimp the edges of the pie crust closed.

In a small bowl, beat the egg and the milk for the eggwash together. Lightly brush it on the top of the pie crust. If you want, you can sprinkle the top of the pie with granulated sugar. 

Bake for 15 minutes and reduce temperature to 350°F, continue baking for 45-50 minutes. Filling should be bubbly and crust light golden brown. If your pie is not ready after that time, continue checking every half hour and cover crust edges with foil to prevent burning.

Cool for 2-3 hours at room temperature before serving.