Make the Pie Dough (if homemade)
In a large bowl, whisk flour, salt, and sugar.
Cut in the cold butter using a pastry cutter or your fingers until crumbly.
Slowly add ice water, a tablespoon at a time, mixing until dough holds together.
Divide into two discs, wrap each in plastic wrap, and chill for 30 minutes (or up to 2 days).
Prepare the Filling
In a large bowl, combine blackberries, sugar, cornstarch, lemon juice, zest, and salt.
Stir gently and let sit while you roll the dough.
Roll Out the Dough
On a floured surface, roll out one disc to fit a 9-inch pie plate.
Press gently into the pan and trim excess dough.
Assemble the Pie
Pour the berry mixture into the crust-lined pie pan.
Roll out the second dough disc and create a lattice top (or full crust with slits).
Crimp the edges to seal and give that classic homemade pie look.
Add Egg Wash
Whisk together egg and milk.
Brush the crust lightly and sprinkle with coarse sugar.
Bake the Pie
Preheat oven to 400°F (200°C).
Place pie on a baking sheet and bake for 20 minutes.
Reduce oven to 350°F and bake another 30–35 minutes until golden brown and bubbling.
If browning too quickly, tent loosely with aluminum foil.
Cool and Serve
Let pie cool at least 3 hours so filling sets properly.
Slice and serve with a scoop of vanilla ice cream.