Preheat oven to 350°F and grease a 10x15x1 jelly roll pan (or 9x13 baking dish for thicker bars).
Whisk together dry ingredients in a medium-sized bowl: flour, baking powder, spices, baking soda, and salt.
Cream butter and sugar in a stand mixer using the paddle attachment until light and fluffy. Add eggs one at a time and beat until incorporated. Turn the mixer to low and incorporate vegetable oil, pumpkin puree, and vanilla.
Add the dry mixture into the wet mixture in two batches and mix on low until just combined.
Pour the batter into the prepared pan.
Bake for 25 to 35 minutes, depending on the size of your pan until a toothpick comes out clean. Place on a wire cooling rack to cool completely before frosting.
Frosting in the bowl of a stand mixer fitted with the paddle attachment beat cream cheese and butter until light and creamy, about 3 minutes. Add confectioners sugar, vanilla, and 1 tablespoon of milk to the bowl, mix starting at a low speed, and increase speed as necessary to achieve desired consistency. Add additional milk to thin out the frosting.
Spread the frosting over the pumpkin bars, cut, and serve!