Preheat your oven to 350°F and line a 9x13-inch baking pan with parchment paper, leaving slight overhang for easy removal. This prevents sticking and creates clean edges on your finished bars.
Whisk together flour, baking powder, baking soda, cinnamon, ginger, cloves, and salt in a large bowl, breaking up any lumps in the spices. Set the dry mixture aside while you prepare the wet ingredients for your pumpkin bars mixture.
In another bowl, whisk pumpkin puree, sugar, oil, eggs, and vanilla extract until smooth and fully combined. This step takes about one minute of vigorous whisking to ensure even distribution.
Fold the dry ingredients into the pumpkin mixture using a spatula, stirring just until combined—do not overmix. Overmixing develops gluten and creates tough bars instead of tender ones.
Pour the batter into your prepared pan, spreading it evenly to the edges with a spatula. Bake for 28-32 minutes until a toothpick inserted in the center comes out clean or with just a few moist crumbs.
Remove from the oven and cool completely on a wire rack before frosting; this prevents the frosting from melting into the warm bars. The fall pumpkin bars crowd recipe requires patience here for best results.
Beat softened butter with cream cheese frosting and powdered sugar until light and fluffy, then spread evenly across cooled bars. Cut into 24 squares and serve immediately or store for later entertaining occasions.