Using a stand mixer or a hand mixer, cream butter for about 3 minutes, until light and fluffy. Slowly add powdered sugar and beat until combined. Add rum extract (if using) and eggnog, one tablespoon at a time until desired consistency or flavor is reached.
Taste and add cinnamon, nutmeg, and rum extract, if desired. Beat for an additional two minutes smooth. Frost cooled cookies and add sprinkles if desired.
Adapted from Taste of Home