In a large bowl, mix all purpose flour and salt together. Using a pastry cutter or a standing mixer (on low), cut the unsalted cubed butter into flour, mixing until it resembles coarse crumbs (the butter pieces should be about pea sized at this stage).

Add in the ice water, 1 Tbsp at a time, until the dough just comes together. (I used approx. 9 Tbsp).

Turn the dough out onto a floured surface and shape the dough into 2 balls. Flatten each into the shape of a disc and wrap in plastic wrap.

Chill for at least 30 minutes before using.

Preheat the oven to 400 degrees and set out your 9” or 10” deep dish pie plate.

Mix blackberries and granulated sugar together. Using a fork, mash about 1/2 of the berries just until the begin to break apart. Mashing some and leaving others whole gives the filling a nice texture.

Add the lemon zest, lemon juice, cornstarch, and flour. Gently mix until combined.

Set aside.

On a floured surface, roll out 1 pie dough disc into a large circle, about 1/4” thick.

Loosely wrap the dough around the rolling pin and gently transfer to your pie plate. Gently unwrap the dough from the rolling pin and set the crust in the pie plate.

Cut off any excess dough, making sure to leave about 1/2” of overhang (this will help you to have enough dough to crimp the edges of your pie).

Pour half of the blackberry filling into the pie crust. Stud the butter cubes around the pie. Then, pour the rest of the filling on top. Set aside.

Roll out the remaining pie disc into a large circle, about 1/4” thick. Using a knife or pizza cutter, slice the dough into long strips. You will need 10 strips (if you can’t get 10 strips then you’ll need to take the scraps of the dough and re-roll and cut).

Weave the pie dough strips to make a woven pattern across the pie. I place 5 strips on the pie first, then I weave the remaining 5 pie strips until I have a woven lattice crust.

Tuck the edges of the pie dough (including the lattice edges) underneath the bottom of the crust. Crimp the edges with a fork or with your knuckle and thumb.

Beat the egg and water together, then brush the egg wash on top of the pie. This will give the pie a nice golden color.

Bake at 400 degrees for 20 minutes.

Reduce heat to 375 degrees and continue to bake for 45-50 minutes, or until the blackberry filling is bubbling and the crust is golden. If the crust is browning too quickly, use a pie shield to cover the edges or tent a piece of foil on top.

Allow the pie to cool about 4 hours before slicing (if you cut into it too soon, the filling will not be thicken and will run everywhere).