2 refrigerated, store-bought, 9-inch pie crusts, unbaked
Vanilla ice cream, for serving
Place a rack in the center of the oven and preheat the oven to 425 degrees F. Rub a pottery or glass 2-quart baking dish with 1 tablespoon of the butter. Set the dish aside.
Place the blackberries, 3/4 cup of sugar, cinnamon, if desired, and the flour in a medium-size mixing bowl and gently stir to combine. Spoon half of the blackberry mixture into the prepared dish, and dot it with 1 tablespoon of butter.
Roll out the pie crusts and cut into 1-inch strips with a pizza cutter or sharp knife. Arrange half of the strips in a lattice pattern on top of the blackberries. Cover the baking dish with foil, and place it in the oven to bake 15 minutes.
Remove the dish from the oven, and remove the foil. Place the remaining blackberry mixture on top of the crust, dot it with the remaining 1 tablespoon butter, and arrange the remaining strips of crust on top. Sprinkle with the remaining 1 tablespoon sugar. Place the dish in the oven.
Bake until the top has lightly browned, and the blackberries are bubbling in the center, about 15 to 20 minutes. Remove the dish from the oven, and let it cool 10 minutes. Spoon into bowls and serve warm with ice cream.