Preheat the oven to 350 degrees. Spray a 9 x 9 metal baking pan with non stick spray and set aside.
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 ¼ cups (281 g) all purpose flour, spooned and leveled, 1 tablespoon ground pumpkin pie spice, 1 ½ teaspoon baking powder, ¼ teaspoon baking soda, ½ teaspoon 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 tablespoon (140 g) unsalted butter, softened, 1 ½ 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 teaspoon vanilla
Add in the buttermilk and canned pumpkin puree and mix until combined. (It will look curdled at this point but don't worry. Everything will combine together nicely once the dry ingredients are added!) ½ 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 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 from the center with just a few moist crumbs.
Then let the cake cool in the pan on a wire rack until completely cool.