Preheat oven to 350 degrees F. Grease a 9x13-inch baking pan; set aside.
Make the Cake: In a medium bowl, whisk together the flour, cinnamon, baking powder, baking soda and salt; set aside.
In a large bowl, whisk together the pumpkin, sugar, oil and eggs until smooth and thoroughly combined. Pour the flour mixture over the pumpkin mixture and whisk or stir gently until incorporated and smooth.
Pour the batter into the prepared baking dish, smoothing out the top with a spatula.
Bake for 30 minutes or until a thin knife inserted into the center comes out clean. Remove from the oven and cool completely before frosting.
Make the Frosting: Using an electric mixer, beat the butter, powdered sugar, vanilla, and salt on low speed until smooth, then mix for an additional 2 minutes, scraping down the bowl as needed. Increase the speed to medium-low and add the cream cheese one piece at a time and mix until smooth, then mix for an additional 2 minutes.
Spread the frosting evenly over the cooled cake. Cut into squares and serve. Store leftovers covered, at room temperature for up to 1 day or in the refrigerator for up to 1 week.
Notes
Pan: A 9x13-inch metal pan is recommended, though glass will work, too. If you prefer a thinner, denser pumpkin bar, you can bake these in a 10x15-inch jelly roll pan and reduce the baking time (start checking for doneness around 20 minutes).
Canned Pumpkin: Be sure to purchase the pure pumpkin puree, NOT the pumpkin pie mix, which includes sugar and seasonings.
Cinnamon: You can add pumpkin pie spice if you want additional flavors, such as nutmeg, ginger, cloves, etc. Reduce cinnamon to 1½ teaspoons and add 1½ teaspoons of pumpkin pie spice.
Canola Oil: You can substitute vegetable oil or any other neutral-flavored oil, such as grapeseed oil, corn oil, or sunflower oil.
Optional Add-Ins - While I make these pumpkin bars as-is, you can add 1 cup of add-ins like chocolate chips, chopped nuts, or coconut.
Make Ahead: I always make these the day before I plan to serve! For one day, you can keep them at room temperature, then store them in the refrigerator, covered tightly with plastic wrap.
Make-Ahead Frosting – The frosting itself can be made up to 5 days in advance and stored in an airtight container in the refrigerator. It will firm up in the fridge, so allow it to come to room temperature before spreading on the bars.
Storage: Keep frosted pumpkin bars covered at room temperature for up to 1 day or in the refrigerator for up to 1 week in an airtight container or covered with plastic wrap.
Freezing: These bars freeze wonderfully! Place in an airtight container (if you need to stack them, place a piece of parchment paper between layers) and freeze for up to 3 months. Thaw in the refrigerator overnight and serve cold or bring to room temperature before serving.