Preheat oven to 325ºF.
To prepare crust, roll dough into a 14-inch circle; fit into a 9-inch deepdish pie plate coated with cooking spray. Fold edges under; flute. Place pie plate in refrigerator until ready to use.
To prepare streusel, lightly spoon 1/3 cup flour into a dry measuring cup; level with a knife. Combine 1/3 cup flour, brown sugar, and 1/2 teaspoon cinnamon in a medium bowl; cut in butter with a pastry blender or 2 knives until the mixture resembles coarse meal. Place streusel in refrigerator.
To prepare the filling, heat a large nonstrick skillet coated with cooking spray over medium heat. Add sliced apple, 1/4 cup granulated sugar, and 1/2 teaspoon cinnamon; cook 10 minutes or until the apple is tender, stirring mixture occasionally. Spoon the apple mixture into prepared crust.
Combine remaining 3/4 cup granulated sugar, 2 tablespoons flour, salt, and eggs, stirring with a whisk. Stir in buttermilk and vanilla. Pour over apple mixture. Bake at 325ºF for 30 minutes. Reduce oven temperature to 300ºF (do not remove pie from oven); sprinkle streusel over pie. Bake at 300ºF for 40 minutes or until set. Let stand 1 hour before serving.