In the bowl of a stand mixer fitted with the paddle attachment, beat butter at medium speed until creamy, about 1 minute, stopping to scrape sides of bowl. With mixer on low speed, gradually add confectioners’ sugar, about 1 cup (120 grams) at a time, alternately with eggnog, about 1 tablespoon (15 grams) at a time, beating just until combined after each addition. Beat in nutmeg and vanilla. Increase mixer speed to medium, and beat until light and fluffy, about 2 minutes, stopping to scrape sides of bowl. Use immediately.