Preheat oven to 350º and grease a 9"-x-9 baking pan with cooking spray. Set aside.
Next, prepare snickerdoodle layer. In a medium bowl, whisk together flour, baking soda, cinnamon, and salt. In a separate large bowl or stand mixer fitted with the paddle attachment, cream the butter and sugars until pale and fluffy, about 3 minutes. Add eggs and vanilla and mix until just combined. Gradually add prepared flour mixture and mix on low until just combined.
Press two-thirds of the snickerdoodle dough into an even layer in the bottom of the pan and sprinkle with cinnamon sugar. Bake in preheated oven for 15 minutes.
While the dough is baking, make the apple pie filling. Combine the sliced apples, flour, granulated sugar, and cinnamon together in a large bowl until all of the apples are evenly coated. Once the bottom layer is baked pour the apple mixture on top and press down firmly.
Break up remaining snickerdoodle dough into small clumps and sprinkle over apple pie layer, then sprinkle with more cinnamon sugar.
Place back in the oven until it turns golden brown, about 35-40 minutes. Let cool completely before slicing into squares.