Before you begin, make sure the butter and cream cheese are at room temperature.
In a stand mixer with a paddle attachment, whip the room temperature butter on medium-high speed for five minutes. You will see a dramatic change-it will go from yellow to bright white and fluffy.
Scrape down the sides of the bowl and add in the cream cheese. Mix for another two minutes to completely combine.
Sift the powdered sugar and add into the mixer in two batches, scraping the sides of the bowl in between. Let the mixer run for about 4 minutes and the frosting will be thick and velvety.
Add the vanilla extract, 1 tablespoon of milk and mix to combine. Check the consistency with a spatula. If you prefer to use a spatula to decorate the cake, you may want to add the reserved 1 tablespoon of milk for a more spreadable consistency. For best results, use immediately.