1 1/2 tablespoons unsalted butter, cut into small pieces
Preheat the oven to 350°. Set a baking sheet lined with foil on the bottom rack. In a large bowl, toss 1 half-pint of the blackberries with the sugar, cornstarch, almond extract and salt; lightly crush the berries. Fold in the remaining berries and let stand for 5 minutes.
Spoon the fruit and any accumulated juices into a 9-inch glass pie plate lined with 1 round of the Flaky Pie Crust. Dot the filling with the butter. Lightly brush the pastry rim with water. Position the rolled-out top crust over the filling and press the edges together to seal. Trim the overhang to 1/2 inch, fold it under itself and crimp decoratively.
Bake the pie in the center of the oven for 1 1/2 hours, or until the filling is bubbling and the crust is deep golden. Cover the edge with strips of foil halfway through baking to prevent overbrowning. Transfer to a wire rack and let cool for at least 4 hours before cutting.