Add the butter and vanilla extract to the bowl of an electric stand mixer, and beat until smooth and creamy.
Sift in about 1 cup of the powdered sugar, and add a splash of milk. Mix until combined.
Repeat 2 more times, until all of the powdered sugar and milk has been incorporated. If it needs loosening up, add more milk. If it's too runny, sift in more powdered sugar.
Finally, beat the frosting on a medium speed for 5 minutes, and use as desired.