In a stand mixer with a whisk attachment, cream the butter and cream cheese together until well combined.
Add in the powdered sugar, vanilla extract and milk. Whisk on medium speed until fluffy (this will take roughly 3-5 minutes). If the icing looks too thick, add small amounts of milk (1/2 TBSP at a time) to soften the consistency.
Once fluffy, use or store away in an airtight container for later use.