In a large bowl, combine the flour, sugar, and salt.
Add the chilled butter pieces and use a pastry cutter or your fingers to work the butter into the flour mixture until it resembles coarse crumbs.
Gradually add ice water, 1 tablespoon at a time, until the dough comes together. Be careful not to overwork it.
Divide the dough into two equal portions, flatten into disks, and wrap in plastic wrap. Chill in the refrigerator for at least 1 hour.
In a large bowl, combine the Saskatoon berries, sugar, cornstarch, lemon juice, lemon zest, vanilla extract, and cinnamon (if using). Toss gently to coat the berries evenly.
Preheat your oven to 400°F (200°C).
On a lightly floured surface, roll out one disk of dough to fit a 9-inch pie pan. Carefully transfer the rolled dough to the pan and trim the edges.
Pour the berry filling into the pie crust.
Roll out the second disk of dough and place it over the filling. You can use it as a whole crust or cut into strips for a lattice top. Trim and crimp the edges to seal.
If using a lattice top, brush the crust with the beaten egg and sprinkle with granulated sugar.
Place the pie on a baking sheet to catch any drips.
Bake for 45-50 minutes, or until the crust is golden brown and the filling is bubbly.
Allow the pie to cool completely before slicing to let the filling set.
Enjoy your meal!