Preheat the oven to 350°F. Line a square baking pan with parchment paper and set aside.
In a medium bowl, whisk the flour, cinnamon, baking soda, and salt together. Set aside.
Using a hand mixer or a stand mixer fitted with a paddle attachment, cream the butter with the brown sugar until smooth. Add the pumpkin and vanilla extract and beat to incorporate. Add the dry ingredients and mix on low speed until combined.
Transfer the batter to the prepared pan. Use a rubber spatula to smooth out into an even layer. The batter will be thick and slightly sticky. Bake for 15-18 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool in the pan completely before frosting.
To make the cream cheese frosting:
Using a hand mixer or a stand mixer fitted with the paddle attachment, beat the cream cheese and butter together until smooth and creamy. Add the powdered sugar and vanilla extract and continue beating until combined. Spread onto cooled bars before slicing into individual squares.
Be sure to use 100% pumpkin puree, not pumpkin pie filling, which contains added ingredients.
For the frosting, I recommend a full-fat block of cream cheese. Do not use the spreadable cream cheese that come in a tub.