Lemon juice and zest give this simple buttercream frosting a refreshing zing.
Beat butter with a mixer on medium-high speed until pale and creamy, about 2 minutes. Reduce speed to medium-low. Add sugar, 1/2 cup at a time, beating and scraping sides of bowl after each addition, about 5 minutes total. Add lemon juice and zest, and beat until frosting is smooth. Use immediately, or cover and refrigerate for up to 3 days. (Bring to room temperature, and beat on low speed until smooth before using.)