Preheat oven to 325°F.
To prepare streusel, combine flour, brown sugar and cinnamon in a small bowl, cut in butter with a pastry blender until the mixture resembles coarse meal. A mini-chopper is handy for this if you have one. Store in refrigerator until ready for use.
For filling: heat a large nonstick skillet coated with cooking spray over medium heat. Add sliced apples, 1/4 cup granulated sugar and cinnamon. Cook for about 10 minutes, or until apples begin to soften. Stir occasionally. Spoon apple mixture into prepared crust.
Combine remaining 3/4 cup granulated sugar, 2 tbsp. flour, salt and eggs, stirring with a whisk. Stir in buttermilk and vanilla. Pour over the apple mixture. Bake at 325°F for 30 minutes. Reduce the oven temperature to 300°F (do not remove from oven); sprinkle streusel over pie. Bake at 300°F for 40 minutes or until set. Let stand 1 hour before serving.
