Preheat your oven to 350 F. Grease and line three 6” round cake pans.
In an electric mixer fitted with the paddle attachment, beat the butter, oil, granulated sugar, and light brown sugar on medium-high speed for 2-3 minutes, until light and fluffy. Make sure to occasionally scrape down the sides of the bowl.
Add the egg and vanilla, and continue beating on medium for another 2 minutes.
Reduce speed to low, and gradually add the buttermilk and pumpkin puree, then increase the speed and continue beating until smooth and combined. If your buttermilk is cold, make sure to give it a quick zap in the microwave to bring it to room temperature.
In a separate bowl, whisk the flour, baking soda, baking powder, salt, and all the spices together until combined.
Add all of the flour at once into your mixer and mix on the lowest speed possible until the flour is just combined. You could alternatively fold the flour in by hand. Do not overmix.
Divide the batter between the cake pans and bake for 30-33 minutes, or until the cake is set.
Let the cake rounds cool in the pans for 10 minutes before transferring them to wire racks to cool completely.