Preheat oven to 375°F. Spray a 9-inch round baking dish with cooking spray.
Evenly distribute the blackberries on the bottom of the baking dish. Mix 2 tablespoons of flour with the brown sugar, and sprinkle it over the blackberries. Add the vanilla to the melted butter, then drizzle it over the berries.
Topping
Add the flour, granulated sugar, and salt to a food processor. Pulse to blend.
Cut the butter into 8 pieces and place (spread out) on the flour mixture. Process until the mixture forms a dough. (If you do not have a food processor, work the mixture by hand or with a mixer.)
Break the dough into small pieces and place over the blackberries.
Bake for 40 minutes or until the topping is golden brown. If necessary, place a sheet of aluminum foil over the pie during the last 10 minutes of baking to prevent overbrowning.