In small bowl, whisk together the flour, baking powder, baking soda, spices, and salt. Set aside.
In large bowl, beat together the sugars, eggs, pumpkin, oil, and vanilla for about 60 seconds. Add the dry ingredients and mix just until smooth.
Pour into a greased 12” X 17” (half sheet pan), rimmed sheet pan. See notes.
Bake at 350°F for 18 to 20 minutes, rotating after 10 minutes. Bars are done when they’re just turning brown around the edges and pulling away from the sides of the pan, and a toothpick inserted in the middle comes out clean. Be careful not to overbake or the bars will be dry.
Cool completely before frosting.