In the bowl of your stand mixer, cream butter until smooth using whisk attachment.
Add eggnog, salt, vanilla, nutmeg, and one cup of powdered sugar. Blend until combined. Scrape down sides. Repeat process, adding one cup of sugar at a time, until desired consistency is achieved. Story in refrigerator until ready to use.
When frosting is cold, it will hold its shape, but at room temperature it may not. Frosting is very sweet.