Preheat oven to 350 degrees. Grease a 10-inch by 15-inch jelly roll pan.
In a small bowl, whisk together flour, baking powder, baking soda, cinnamon, pumpkin spice, and salt. Set aside.
In a large bowl, combine sugar, pumpkin, and oil. Mix well. Whisk in eggs.
Fold in the flour mixture until uniformly combined. Spread into prepared pan. Bake for 20 to 25 minutes, until a toothpick inserted comes out clean. Cool completely, at least 30 minutes.
Using a standing mixer with the paddle attachment, or using an electric hand mixer with a large bowl, combine cream cheese, butter, and vanilla until smooth. Add the powdered sugar in batches until combined. Spread on the cooled cake.
Cut into 24 bars and top each with a MellowCreme pumpkin.