Preheat oven to 350°F. Grease and flour a 9 inch square baking pan or line with nonstick foil. Alternatively, you can use a 7x11 inch baking dish
With an electric mixer, beat together oil, eggs, sugar, pumpkin, baking powder, salt, and spices.
Add the flour and stir until blended.
Pour batter into prepared pan and spread evenly.
Bake until a toothpick inserted into the center comes out clean -- about 30 minutes. Let cool completely before frosting.
In a large bowl, beat softened cream cheese and butter until well combined. Add powdered sugar and mix until creamy, then beat until fluffy (it's fairly stiff). Beat in vanilla.
Spread cream cheese over bars. If desired, sprinkle with nuts and crystallized ginger.
Cut into 16 bars.