Melt 60g butter in a frying pan over medium heat. Add apple, caster sugar and cinnamon. Cook 3 to 5 minutes, until tender. Set aside.
For the custard:
In a large mixing bowl combine 60g softened butter with 75g sugar. Beat until creamy. Add eggs one at a time, beating until pale and the yellow disappears. Mix in vanilla, then 2 tablespoons flour. Combine thoroughly, then pour in buttermilk, beating until smooth.
Prick bottom of pastry case with a fork. Spoon in apple mixture, then pour custard over it.
Place in preheated oven and bake for 1 to 1 1/4 hours, until a knife inserted in the centre comes out clean. Let cool for 1 hour before slicing.