Gradually add the butter, 1 piece at a time as the mixer runs, beating on medium-high speed. Let each piece of butter incorporate fully before adding another. The texture of icing the frosting will change as the butter is incorporated.: First, it will become runny. Next, it may appear curdled. Once all of the butter has been added, add the 2 teaspoons of vanilla and continue whipping for about 1-2 minutes until smooth.