1 cup (2 sticks) unsalted butter, room temperature
6 to 8 cups confectioners’ sugar
1/2 cup milk
1 teaspoon pure vanilla extract
In the bowl of an electric mixer fitted with the paddle attachment, cream butter until smooth and creamy, 2-3 minutes. With mixer on low speed, add the sugar one cup at a time, alternating with milk and vanilla. Mix until light and fluffy, and if necessary add the remaining two cups of sugar.
Credit for this goes to http://www.marthastewart.com/336452/billys-vanilla-buttercream this is by far the BEST butter cream recipe I have ever used for butter cream!