Preheat oven to 350º. Grease and flour a 12 x 18 jelly roll pan (baking sheet with a rim).
In a mixer, combine sugar and oil and mix well. Add the eggs, one at a time, and beat well. Next, beat in the pumpkin.
Combine the baking powder, baking soda, salt, spices and flour in a bowl with a whisk. Gradually add the dry ingredients to the mixer, blending well after each addition, until all is mixed well and and batter is smooth.
Spread batter evenly into prepared pan. Drop pan on the counter a few times to release air bubbles. Bake for 25 to 30 minutes, until evenly golden, but not dry. Let cool for at least 1½ hours.
Make frosting by beating together cream cheese, butter, milk and vanilla until smooth. Gradually beat in the powdered sugar. Spread the frosting over the completely cooled bars.
Chill or freeze before cutting into bars of desired size. Keep refrigerated.