1. Preheat your oven to 350°F (175°C). Grease a 9×13 inch baking dish or line it with parchment paper for easy removal.
2. In a medium bowl, whisk together the flour, baking soda, salt, and pumpkin pie spice. Set aside.
3. In a large bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy, about 3-4 minutes.
4. Add the pumpkin puree, egg, and vanilla extract to the butter mixture, and mix until well combined.
5. Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Be careful not to overmix.
6. Fold in the chocolate chips and walnuts, ensuring they are evenly distributed throughout the batter.
7. Spread the batter evenly into the prepared baking dish, smoothing the top with a spatula.
8. Bake for 25-30 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out clean or with a few moist crumbs.
9. Allow the bars to cool completely in the pan before cutting them into squares and serving.

Remember, patience is key. Letting the bars cool ensures they hold their shape when sliced, giving you those perfect, Instagram-worthy edges.