Preheat the oven to 350°F. Grease a 9×9 inch light metal baking pan and line it with parchment paper.
In a medium bowl, whisk together the all purpose flour, pumpkin pie spice, baking powder, baking soda, and salt. Then set aside the flour mixture.
2 1/4 cups (281 g) all purpose flour, spooned and leveled, 1 tbsp pumpkin pie spice, 1 1/2 tsp baking powder, 1/4 tsp baking soda, 1/2 tsp salt
Add the softened butter and granulated white sugar to a large bowl. Cream together for 2 minutes with an electric mixer on high speed. (You can also use a stand mixer.)
10 tbsp (140 g) unsalted butter, softened, 1 1/2 cups (300 g) granulated white sugar
Then add in the eggs and vanilla and mix on medium speed until pale in color and smooth, about 1 minute.
2 eggs, at room temperature, 1 tsp vanilla extract
Add in the buttermilk and canned pumpkin puree and mix on medium-low speed until combined.(It will look curdled at this point but don't worry. Everything will combine together nicely once the dry ingredients are added!) 1/2 cup (120 ml) buttermilk, at room temperature, 1 cup (244 g) canned pumpkin puree, at room temperature
Add the dry ingredients to the wet ingredients and mix on low speed until the cake batter is combined. Scrape the sides of the bowl with a rubber spatula as necessary.
Pour the batter into the prepared baking pan. Bake the cake for 35-38 minutes, or until a toothpick comes out clean from the center.
Let the cake cool in the pan on a wire rack for 30 minutes, then pull up on the parchment paper to release it from the pan. Let the cake continue cooling on the cooling rack until completely cooled.