In a mixing bowl, combine flour, salt, and sugar. Add cold, cubed butter and mix until the mixture resembles coarse crumbs.

Gradually add ice water, one tablespoon at a time, mixing until the dough comes together. Divide the dough in half, shape into disks, wrap in plastic, and refrigerate for at least 1 hour.

In a large bowl, combine blackberries, sugar, cornstarch, lemon juice, and vanilla extract. Gently toss to coat the berries evenly.

Preheat the oven to 425?F (220?C). Roll out one disk of dough on a floured surface to fit the pie pan. Place the rolled dough into the pie pan.

Pour the blackberry filling into the crust and dot with butter. Roll out the second disk of dough and place it over the filling. Seal and flute the edges, then cut slits in the top crust for steam to escape.

Place the pie on a baking sheet to catch any drips. Bake in the preheated oven for 40-45 minutes or until the crust is golden brown and the filling is bubbly.

Allow the pie to cool for at least 2 hours before slicing to let the filling set.