Very easy, but very good:
1 box carrot cake mix
4 tsp pumpkin pie spice
1 can pumpkin
1 block cream cheese
1 stick butter
1 tbsp vanilla
1 box confectioner's sugar
Prepare cake mix as directed on box, then add pumpin pie spice and canned pumpkin. Place in a greased 11x15" (jelly roll) pan. Bake at 350 degrees for 30 min. (will be quite moist) Cool completely. Beat together softened cream cheese, butter and vanilla with electric mixer, then gradually add confectioner's sugar, and continue beating until smooth. Frost completely cooled bars, and store in fridge. (This will also work with spice cake mix, but we prefer the carrot.)
This is my family's favorite autumn dessert. Hope you enjoy it!