Apple Filling: Melt butter in a frypan over medium heat. Add apple, sugar and cinnamon. Cook 3 to 5 minutes until tender. Set aside.
Buttermilk Custard: In a large mixing bowl combine softened butter and sugar. Beat until creamy. Add eggs one at a time beating until yellow disappears.
Mix in vanilla then flour. Combine thoroughly then pour in buttermilk beating until smooth.
Apple Pie: Spoon apple mixture into crust then pour buttermilk custard over it.
Place in preheated oven and bake for 30 minutes.
Crumble Topping: While pie is baking combine caster sugar, brown sugar, flour and cinnamon in a small bowl. Mix in butter with fingertips until mixture is crumbly.
Remove pie from oven after 30 minutes and sprinkle crumble topping over custard. Return to oven and bake for an additional 40 to 50 minutes. Test to see if it is done by inserting a knife in the centre. When the knife comes out clean remove from oven. Let stand 1 hour before serving.