Cream the butter and cream cheese in a stand mixer with a whisk attachment for 3-5 minutes
Add the powdered sugar, and vanilla and mix. Be careful as to not fling powdered sugar everywhere
Add the milk in, a tablespoon at a time. One you creachthe thinkness you desire, whip for 3-5 more minutes. (I typically use 3-4 table spoons for cakes, and cinnamon rolls)