Preheat the oven to 350 degrees F. Line an 8x8" baking pan with parchment paper.
In a mixing bowl, combine the butter (or vegan butter), brown sugar, and pumpkin puree. Mix until well combined.
Mix in the vanilla extract and eggs
Add the gluten-free all-purpose flour, pumpkin spice, baking powder, baking soda, and salt on top of the liquid ingredients.
Mix the dry ingredients lightly, then fold them into the liquid ingredients just until combined (making sure not to overmix).
Pour the batter into the prepared baking pan, spreading it evenly.
Bake in the preheated oven for 25-28 minutes, or until a toothpick inserted into the center comes out clean.
While the bars are baking, prepare the icing. In a mixing bowl, beat together the butter, cream cheese, and vanilla until smooth.
Gradually add the powdered sugar, beating until the icing is creamy and well combined.
Once the pumpkin spice bars have cooled completely, spread the icing evenly over the top.
Cut into squares and serve. Enjoy!