4 sticks unsalted butter, at room temperature
1/2 teaspoon salt
5 cups confectioners' sugar
1/4 cup whole milk
1 tablespoon pure vanilla extract
Beat the butter and salt in a large bowl with a mixer on medium-high speed until smooth and fluffy, about 2 minutes.
Reduce the speed to low and beat in the confectioners' sugar 1 cup at a time, increasing the speed to medium high. Repeat with the remaining 4 cups. Beat until fluffy, about 3 minutes. Beat in the milk and vanilla until smooth.