In the bowl of a stand mixer using the paddle attachment (or in a large bowl using a handheld electric mixer), beat butter on medium-high speed until smooth.
1 ½ cups unsalted butter
Add vanilla and salt and beat on medium-high speed until blended.
1 Tablespoon vanilla extract, ⅛ teaspoon fine sea salt
Add sugar slowly – one cup at a time – beating well on medium speed and scraping the bowl often. When all sugar is added the frosting will be slightly dry.
5 cups confectioners’ sugar
Add eggnog and beat on high speed until the frosting is light and fluffy.
3 Tablespoons eggnog
Notes
Storage: Store buttercream frosting in an airtight container in the fridge for up to a week. If you plan to refrigerate the frosting before using to decorate a dessert, let the frosting come to room temperature and briefly re-whip it with a stand mixer or handheld electric mixer so that it's soft and fluffy before using.Cakes or other desserts that have been decorated with eggnog buttercream frosting can be stored in an airtight container at room temperature for up to 3 days or in the fridge for up to 5 days.