Preheat oven to 350º and grease a 9"-x-9 baking pan with cooking spray. Set aside.
Make the snickerdoodle layer. In a medium bowl, whisk together flour, baking soda, cinnamon, and salt.
2 1/4 cups all-purpose flour, 1/2 teaspoon baking soda, 1 1/2 teaspoons ground cinnamon, 1 teaspoon kosher salt
In a separate large bowl or stand mixer fitted with the paddle attachment, cream the butter and sugars until pale and fluffy, 2-3 minutes. Add eggs and vanilla and mix until just combined. Gradually add prepared flour mixture and mix on low until just combined.
3/4 cup unsalted butter, 1 cup sugar, 1/2 cup brown sugar, 2 large eggs, 1 teaspoon pure vanilla extract
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.
2 large apples, 2 Tablespoons all-purpose flour, 2 Tablespoons granulated sugar, 1 teaspoon ground cinnamon
Break up remaining snickerdoodle dough into small clumps and sprinkle over apple pie layer, then sprinkle with more cinnamon sugar.
cinnamon sugar for sprinkling
Place back in the oven until it turns golden brown, about 35-40 minutes. Let cool completely before slicing into squares.