Have your 1 2-crust pie crust ready in the fridge and butter a 9-inch pie pan. Preheat the oven to 375℉, and let the dough sit at room temperature for about 10 minutes or until easy to work. First, roll out one disk of the dough to a 12-inch circle, then fit it into the pie pan. Roll out the other disk into an 11-inch circle.
In a medium bowl, gently stir together the 1 ½ pounds blackberries, ½ cup granulated sugar, 3 tablespoons cornstarch, 1 tablespoon fresh lemon juice, and 1 teaspoon pure vanilla extract. Pour into the bottom pie crust and scatter the butter cubes over the top.
Add the top crust dough, and press the edges of the dough together. Trim away any excess to 1 inch of overhang, then fold the edge under and crimp. Cut four slits into the top crust.
Place the pie on a baking sheet and place it in the oven. Bake for 1 hour and 15 minutes, or until the top crust is browned and the fruit filling is bubbling. Cool on a wire rack for at least 2 hours, preferably at least 4, before slicing.