Step 1Melt 2 tablespoons of the butter in a skillet. Add the apples, one-fourth cup sugar and the cinnamon. Cook over medium heat until the apples are tender, 4 or 5 minutes. Set aside.
Step 2In a large bowl, combine the remaining 4 tablespoons of butter with the remaining one-half cup sugar and beat until creamy. Beat in the eggs 1 at a time. Mix in the vanilla, flour and buttermilk and beat until silky.
Step 3Prick the bottom of the pie crust with a fork. Spoon the apples into the crust and spread them around as flat as possible. Pour in the buttermilk custard mixture, ensuring that it covers all the apples. Set in a 300-degree oven and bake until a knife inserted in the center comes out clean, 50 to 55 minutes.