Preheat oven to 350°F. Grease a 9x9 inch baking pan.
Whisk together flour, pumpkin pie spice, baking powder, baking soda, and salt.
Cream butter and sugar until fluffy, then add eggs and vanilla.
Mix in buttermilk and pumpkin puree, then add dry ingredients.
Pour batter into prepared pan and bake for 35-38 minutes.
Let cake cool completely.