Preheat oven to 350ΒΊ 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. The dough will be thick and slightly sticky, so spray a rubber spatula with nonstick spray or run it under warm water to prevent sticking. 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.
2 large apples, 2 Tablespoons all-purpose flour, 2 Tablespoons granulated sugar, 1 teaspoon ground cinnamon
Once the bottom layer is baked, arrange the apple mixture evenly over the crust and press down firmly. They will be overlapping.
Break up remaining snickerdoodle dough into small clumps and sprinkle over apple pie layer, then sprinkle with more cinnamon sugar.
cinnamon sugar for sprinkling
Bake for 35-40 minutes, until the top is puffed, set, and golden brown and the edges of the apple filling are gooey and bubbling. If the bars are starting to look to brown before the tops and edges are done, cover with foil to prevent burning.
Let cool completely before slicing into squares so the apple filling as time to set.