Preheat the oven to 300 degrees. Line an 8 x8 baking dish with foil.
In a medium bowl cream together the butter and sugar. Mix in the vanilla, salt, cinnamon, and flour until a dough forms.
Press the dough into the bottom of the prepared pan and bake for 15 minutes. Remove from oven and turn the oven temperature up to 350 degrees.
Meanwhile, combine the apples, flour, sugar, and apple pie spice in a medium bowl. Toss to coat.
Mix together the ingredients for the topping until a crumble forms.
When the shortbread comes out of the oven sprinkle with half of the crumble mixture. Then layer the apples on top of it in 2 rows. Sprinkle with remaining topping.
Bake for 30 minutes.
Remove from the oven and cool on a wire rack for 30 minutes then place in the refrigerator for at least 2 hours.
Remove the baking dish from the refrigerator 15 minutes before serving. Cut into squares and enjoy!