Preheat oven to 350 degrees F. Grease and flour (or spray with cooking oil spray) a 10 x 15-inch baking pan. If you use a more common 9 x 13-inch pan, you will have to cook it longer or the middle will not get done. Alternatively you could only pour in 3/4 of the batter into the 9 x 13 and use a muffin tin or small baking pan for the rest.
In a large mixing bowl or stand mixer, beat together the sugar, the oil and the pumpkin until creamy (about 1 min). Beat in eggs until well combined - about 2 minutes.
In a medium mixing bowl, sift together flour, baking powder, baking soda, salt, and all of the spices. Add these dry ingredients to the pumpkin mixture and mix at low speed until thoroughly combined and the batter is smooth.
Spread the batter into the prepared pan and bake at 350 degrees for 25 to 35 minutes, until evenly golden and a toothpick or knife inserted into the middle of the pan comes out clean or with a few moist crumbs attached. Let cool completely (about 1 1/2 hour) before frosting.
Using a stand mixer, beat together cream cheese, butter, milk and vanilla. Gradually beat in powdered sugar, a little at a time, until mixture is smooth. Be sure and add powdered sugar slowly or it will fly out and make a big mess. Spread frosting over completely cooled bars.
Chill before cutting into bars. Makes 2-3 dozen bars (depending on how large you cut them).