Let your butter sit out for awhile to soften. DO NOT put it in the microwave. If you do not have time, cut the butter up in small chunks.
Beat the butter until soft and creamy in your mixer. If you dont have a kitchen mixer you can do this with a hand mixer but its best to go ahead and left your butter soften.
Add in shortening, beat again. I like to give mine a few minutes on medium high...beating it a little longer makes it lighter.
Add vanilla and mix well.
Add in the powdered sugar about a cup at a time.
Add in milk 1 TBS at a time until youve got your desired consistency.