Preheat oven to 350 degrees. In a large bowl, mix together oil, brown sugar, sugar, eggs, vanilla, and buttermilk for 3-4 minutes until creamy. Add in pumpkin and mix for 1 minute longer.
Fold in flour, pumpkin pie spice, cinnamon, baking powder, baking soda, and salt and mix until most of the bumps disappear.
Place a parchment paper sheet into the jelly roll pan and spray with non-stick cooking spray. If you don't have parchment paper, spray the pan with non-stick cooking spray.Pour the pumpkin batter into the pan and evenly spread into the corners.
Bake for 20-25 minutes, depending on the size of pan used. If using a thin cookie sheet, I suggest setting the timer for 17 minutes and checking it.
Let cool before frosting. If you used parchment paper you can flip the bars onto a cooling rack and peel off the paper. This makes it easier to cut the bars.Make the cream cheese frosting and once the bars cool, spread the frosting all over. Cut into squares.
Cream Cheese Frosting:
In a mixing bowl, add softened butter, softened cream cheese, powdered sugar, and heavy cream. Mix until light and fluffy and completely smooth and creamy.
Notes
Pure Canned Pumpkin -- only use 100% pure pumpkin, NOT pumpkin pie filling
Oil -- use any type of oil such as canola, vegetable, olive oil, etc.
Buttermilk or Whole Milk -- may substitute 2% or whole milk
TIPS:
When you are ready to cut into pumpkin squares, there are a few ways to make it easier.
Freeze the bars for 30 minutes before slicing them to make them easier.
If you use parchment paper sheets, you can lift the bars out of the pan or flip them over onto a cooling rack. This makes it easier to cut them.
Use a plastic knife to cut the bars. It creates a smooth edge.