This recipe will make 4 cups - it can be halved or quartered to make the desired amount.
3 sticks unsalted butter (bring to room temperature)
3 cups confectioners' sugar
2 tbsp. milk
2 tsp. vanilla extract
1/8 tsp. salt
1. Using an electric mixer, beat butter in a large bowl until fluffy, about 30 seconds.
2. With mixer on low speed, add sugar 1 cup at a time, and mix to combine. Increase speed to high and beat until pale and fluffy, about 1 minute.
3. Reduce speed to medium-low and add milk, vanilla, and salt. Increase speed to high and beat until fluffy, 30 more seconds.