In large mixing bowl, combine ¾ cup granulated sugar, brown sugar, oil, eggs, vanilla and pie spice; beat with electric mixer until well combined, scraping sides of bowl often.
Add in pumpkin; continue mixing until well combined.
Reduce mixer to low and slowly add in flour, baking powder, baking soda and salt. Continue mixing until well combined.
Transfer to a 15x10-inch jelly-roll pan that has been sprayed with no-stick cooking spray. Bake in a preheated 350°F oven 25-30 minutes or until a knife inserted in center comes out clean. Remove from oven and cool completely.
In medium bowl, combine softened cream cheese, remaining ½ cup granulated sugar and milk. Beat with electric mixer until light and fluffy. Spread over tops of bars.