Preheat oven to 350.
Grease an 8x8" pan.
In a large bowl, beat together pumpkin, oil and vanilla.
In a separate bowl, sift together flour, sugar, cinnamon, baking powder, and baking soda.
Add dry mixture to wet and mix until just moistened.
Pour into prepared pan and bake for 20-25 minutes.
Meanwhile, in a small bowl, beat butter and cream cheese.
Add vanilla, milk, and powdered sugar, and beat until smooth.
Spread frosting over cooled bars, slice, and serve.