In a mixing bowl, whisk together flour, pumpkin pie spice, cinnamon, baking soda, and salt; set aside.
In the bowl of a stand mixer, or in a large bowl using a handheld electric mixer, cream the butter, applesauce, granulated sugar and brown sugar on medium-high speed until smooth. Beat in egg and vanilla until combined. Add pumpkin puree and mix until combined. Don't worry if the mixture looks curdled - that's completely fine!
Spread the batter evenly in the prepared pan. Top with reserved chocolate chips, if wanted, and bake for 35-40 minutes. The edges will begin to pull away from the sides of the pan and will be golden brown and the center will look "puffy" and done. A toothpick inserted in the middle should come out with moist/dry crumbs.
You do not have to use unsweetened applesauce for these bars. I have used both sweetened and unsweetened applesauce for this recipe, and it's great with either. They will slightly (not by a noticeable difference) more sweet is using sweetened applesauce.
Be sure and use the can of pure pumpkin. DO NOT use the canned pumpkin pie mix.