Preheat the oven to 350°F. Grease a 9×9 pan and line it with parchment paper.
In a medium bowl, whisk together the flour, pumpkin pie spice, baking powder, baking soda, and salt. Set aside.
In a large mixing bowl (or stand mixer), cream the butter and sugar until light and fluffy, about 2 minutes.
Add the eggs one at a time, then the vanilla. Mix until smooth and pale.
Mix in the buttermilk and pumpkin purée.
Add the dry ingredients and mix on low until just combined. Scrape down the bowl as needed.
Pour the batter into your prepared pan and bake for 35–38 minutes, or until a toothpick comes out clean.
Cool in the pan for 30 minutes, then lift out with the parchment and let cool completely on a rack.