Beat the butter and shortening in the bowl of your mixer until blended and creamy. 

Beat in vanilla. 

Gradually add in the icing sugar, one cup at a time, beating well on medium speed and scraping down the bowl. 

Once all the sugar is added, beat until light and fluffy.

For spreadable consistency, add milk (or water or corn syrup) a little at a time until the desired spreadability is reached.