Preheat the oven to 350°F (177°C) and grease a 10×15-inch baking pan. Or line the pan with parchment paper with enough overhang on the sides to easily lift the bars (as a whole) out.
Whisk the flour, baking powder, baking soda, salt, cinnamon, and pumpkin pie spice together in a large bowl. Set aside. Whisk the oil, eggs, brown sugar, granulated sugar, maple syrup, pumpkin, and vanilla extract together until combined. Pour the wet ingredients into the dry ingredients and use a mixer or whisk until completely combined. Batter will be thick.
Spread batter into the prepared pan. Bake for 28-35 minutes. Baking times vary, so keep an eye on yours. The bars are done when a toothpick inserted in the center comes out clean. If you find the top or edges of the bars browning too quickly, loosely tent with aluminum foil.
Remove the bars from the oven and set the pan on a wire rack. Cool bars completely. (After about 45 minutes of cooling, you can place the bars in the refrigerator to speed things up!)
In a large bowl using a handheld or stand mixer fitted with a paddle or whisk attachment, beat the cream cheese and butter together on high speed until smooth and creamy. Add confectioners’ sugar, vanilla, pumpkin pie spice, and salt. Beat on low speed for 30 seconds, then switch to high speed and beat for 2 minutes.
If you want the frosting a little thicker, add an extra 1/4 cup of confectioners sugar. Spread the frosting on the cooled bars. To help “set” the frosting, refrigerate for 30 minutes before slicing and serving. This makes cutting easier and less messy.
Enjoy with or without a fork, but definitely grab a napkin!
Cover leftover bars tightly and store in the refrigerator for up to 5 days. If you skipped the frosting, cover and store plain pumpkin bars at room temperature for up to 3 days or in the refrigerator for up to 1 week.
