In the bowl of a stand mixer or in a large mixing bowl
, beat the butter on low speed with the stand mixer's paddle attachment or on medium-low speed with a hand mixer until the butter is smooth. Add the sugar gradually and continue mixing on the same speed, stopping and scraping the bowl and beater as needed, until lighter in color and slightly fluffy, about 2 minutes. Add the eggs one at a time and mix until smooth after each addition, about 30 seconds. Add the pumpkin and mix until it's evenly incorporated into the batter. Scrape down the sides of the bowl and mix until well blended.