Copyright 2008, reprinted with permission by Little, Brown and Company.
In the bowl of a mixer fitted with a paddle attachment, beat the butter on medium speed until smooth.
Turn the mixer to low speed and add 1 cup of the confectioners' sugar and beat until combined. Scrape the bowl thoroughly.
In a small bowl, combine the milk and vanilla. Add to the butter-sugar mixture. Set the mixer to medium speed and beat until light and fluffy.
Gradually add the remaining confectioners' sugar, adding as much as you need to achieve a smooth and stiff consistency.
Use immediately, store in an airtight container at room temperature for up to 2 days, or refrigerate for up to 10 days.