Spray a 15x10x1" baking pan with cooking spray. Set aside.
For the bars:
Combine the pumpkin, sugar, egg and oil in a medium mixing bowl and mix until well combined.
In a medium bowl combine flour, cinnamon, nutmeg allspice, baking powder, baking soda, and salt. Mix to combine. Add to pumpkin mixture. Mix until fully combined.
Pour batter into prepared baking dish, and spread to cover the bottom of the pan.
Bake for 20-25 minutes or until a toothpick inserted in the center comes out clean.
Cool on a wire rack.
For the frosting:
Using a mixer, combine softened cream cheese, butter, confectioners sugar, vanilla and milk until smooth and creamy.
Frost bars with cream cheese frosting once cool. Cut into bars. Sprinkle with cinnamon if desired.