Fit pie crust into a 9-inch pie plate according to package directions. Fold edges under and crimp.
Melt 1/4 cup butter in a large skillet over medium heat. Add apple slices, 1/2 cup sugar, and 1/2 tsp cinnamon. Cook, stirring occasionally, 3 to 5 minutes or until apple slices are tender; set aside.
Beat 1/4 cup butter and 1 1/3 cups sugar at medium speed with an electric mixer until creamy. Add eggs 1 at a time, beating just until yellow disappears. Add 2 tbsp flour and vanilla, beating until blended. Add buttermilk, beating until smooth.
Spoon apple mixture into pie crust; pour buttermilk mixture over apple mixture.
Bake at 300° for 30 minutes.
Stir together 3 tbsp butter, 1/4 cup sugar, brown sugar, 1/2 cup flour, and remaining 1/4 tsp cinnamon until crumbly. Sprinkle over pie.
Bake 40 more minutes or until a knife inserted in center comes out clean.