Preheat oven to 350 degrees.
Grease an 11″ x 16″ jellyroll pan and set aside.
In a large bowl, mix sugar, eggs pumpkin, oil, and vanilla with an electric mixer until well blended.
Add flour, baking powder, baking soda, cinnamon, and pumpkin pie spice. Mix well.
Stir in nuts (optional).
Pour into greased pan and bake in preheated oven for 20-25 minutes, or until done.
While the batter bakes, mix all frosting ingredients together (adding more or less milk to achieve desired consistency) and set aside.
Once the bars are fully cooled, spread frosting overtop.
Regrigerate after 2 hours or freeze for later.