For apples, melt butter in a skillet.
Stir in apples and 1/2 cup sugar.
Cook, stirring occasionally, until apples are tender - about 5 - 7 minutes.
Set skillet aside for apples to cool to room temperature.
For the custard, beat together butter and sugar. Add eggs, flour, vanilla, and buttermilk - beat until blended.
Place pie crust in a 9 inch pie plate and spoon apple mixture into bottom of crust.
Pour custard over apples.
Bake at 325 for 30 minutes.
For topping, mix together butter, sugar, brown sugar, flour, and cinnamon with a pastry blender or your hands until coarse crumbs form.
Sprinkle topping over pie and bake an additional 30 minutes or until the custard is set.
Allow to cool completely before serving.