Fit piecrust into a 9-inch pieplate according to package directions; fold edges under, and crimp. Prick bottom and sides of piecrust with a fork.
Melt 1/4 cup butter in a large skillet over medium heat; add apple, 1/2 cup granulated sugar, and 1/2 teaspoon cinnamon. Cook, sitrring occasionally, 3 to 5 minutes or until apple is tender; set aside.
Beat 1/4 cup butter and 1 1/3 cups granulated sugar at medium speed with an electric mixer until creamy. A eggs, 1 at a time, beating just until yellow disappears. Add 2 tablespoons flour and vanilla, beating until blended. Add buttermilk, beating until smooth.
Spoon apple mixture into piecrust; pour buttermilk mixture over apple mixture.
Bake at 300º for 30 minutes. Stir 3 tablespoons butter, remaining 1/4 cup granulated sugar, brown sugar, 1/2 cup flour, and remaining 1/4 teaspoon cinnamon until crumbly. Sprinkle over pie. Bake 40 more minutes or until a knife inserted in center comes out clean. Let stand 1 hour before serving.
This is the most amazing pie! I am attempting to make it gluten free as my families diets haved changed in the past year due to celiac disease...including mine! I will let you know how it turns out! In the oven now!