1 cup (1/2 lb.) unsalted butter, at room temperature
4 cups (1 lb.) powdered sugar, sifted
1/8 teaspoon salt
2 tablespoons milk
2 teaspoons vanilla
In a bowl, with a mixer (preferably fitted with whisk attachment) on low speed, beat butter, 2 cups powdered sugar, and the salt until blended. Add milk and vanilla; beat until blended. Add remaining powdered sugar, 1 cup at a time, beating until incorporated. Turn mixer to high speed and beat frosting until fluffy and smooth.
Coffee frosting: Replace milk and vanilla with 4 teaspoons instant espresso powder dissolved in 3 tablespoons hot water, then cooled.
Lemon frosting: Replace milk with 2 tablespoons lemon juice, and vanilla with 2 teaspoons grated lemon peel.
Mint frosting: Replace vanilla with 4 teaspoons crème de menthe or 1 teaspoon mint extract and 2 or 3 drops green food coloring, if desired.
Orange frosting: Replace milk with 2 tablespoons orange juice, and vanilla with 2 teaspoons grated orange peel.