In a medium bowl or stand mixer with whisk attachment, beat butter until light and fluffy (about 1 minute).
Gradually add the powdered sugar one cup at a time (start on low setting), scraping down sides of bowl as you go. Add 3 tablespoons of eggnog and full amount of nutmeg and salt.
Gradually beat in just enough remaining eggnog to make frosting smooth and spreadable. If frosting is too thick, beat in more (a few drops at a time).
Transfer to piping bag with tip (I used Wilton 2D) and frost cooled cupcakes with a swirl of buttercream.
Optional- Garnish each cupcake with crushed candy cane bits or sprinkle of nutmeg.
Notes
Sift the powdered sugar before adding to the butter for light and fluffy frosting.
Use freshly ground nutmeg for the best flavour.
Can I add rum to this frosting? Use up to ¼ teaspoon of rum extract to achieve the classic rum and eggnog flavour.
For garnish, use subtly flavoured candy canes. Place in plastic bag and crush with a rolling pin. You could also use sprinkles or light dusting of nutmeg instead.
Frosting can be made 1 or 2 days in advance. Store in an airtight container in the refrigerator. Before frosting cupcakes, bring to room temperature and stir before filling piping bag.