12 ounces (3 sticks) unsalted butter, softened
1 pound confectioners’ sugar, sifted
1/2 teaspoon pure vanilla extract
Beat butter with a mixer on medium-high speed until pale and creamy, about 2 minutes.
Reduce speed to medium. Add sugar, 1/2 cup at a time, beating after each addition, about 5 minutes. (After every 2 additions, increase speed to high, and beat for 10 seconds, then reduce speed to medium-high).
Add vanilla, and beat until buttercream 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.)
Source: Martha Stewart
makes 4 cups