In a large bowl, combine the flour, salt, and sugar. Add the cold, cubed butter and use a pastry blender or your fingers to mix until the mixture resembles coarse crumbs.

Gradually add ice water, stirring until the dough comes together but is not wet or sticky. Adjust with more flour or water if necessary.

Divide the dough into two halves, form into disks, wrap in plastic wrap, and refrigerate for at least 1 hour.

In a large bowl, gently toss the blackberries with sugar, flour, cornstarch, lemon juice, lemon zest, and cinnamon until the berries are evenly coated. Let sit for about 15 minutes to macerate.

Preheat the oven to 375°F (190°C).

Roll out one disk of pie dough on a floured surface to fit a 9-inch pie plate. Transfer it to the pie plate, pressing gently into the bottom and sides.

Pour the blackberry filling into the crust.

Roll out the second disk of dough and place it over the filling. Trim excess dough, and crimp the edges to seal. Cut slits in the top crust to allow steam to escape.

Brush the top crust with beaten egg and sprinkle with sugar.

Place the pie on a baking sheet to catch any drips. Bake for about 45-50 minutes, or until the crust is golden and the filling is bubbly.

If the edges of the pie start to brown too quickly, cover them with foil.

Let the pie cool on a wire rack for at least 2 hours before serving to allow the filling to set.