In the bow of a stand mixer, cream the butter until smooth and then slowly add in the powdered sugar one cup at a time. Letting each cup fully incorporate before adding the next. Once all the sugar has been added scrape down the bowl.
Starting with 4 tablespoons, add in the eggnog along with the nutmeg cinnamon and rum extract. Mix together and then taste, adding in the additional eggnog until it meets your desired taste. We used 6 tablespoons total of the eggnog. We recommend not adding in more beyond this or your buttercream will become too thin.
Start with mixer on low then slowly increase the speed to high and mix for 5 minutes. Stop the mixer once or twice throughout to scrape the bowl. The buttercream will begin to fluff and create a beautiful smooth buttercream.
Use right away or store in an airtight container in the refrigerator. If you don't use the buttercream that day take it out of the refrigerator and bring to room temperature. Then rewhip with the mixer to fluff the buttercream back up.