Read all the instructions below thoroughly before starting.
In a medium bowl. whisk or sift the powdered sugar until fluffy and almost no lumps remain. Set aside
Add butter and cream cheese to a different mixing bowl and cream them together with a hand or stand mixer until fluffy, about 2 minutes on medium high. Stop to scrape down the sides as needed.
Alternate adding powdered sugar with milk to the cream cheese/butter mix, ending with the milk and stopping the mixer in-between additions, Mix on low and ramp up to medium high with each addition (this is to avoid a big mess...you're welcome).
Add more milk or powdered sugar as needed to achieve the desired spreading consistency.
Add vanilla and the pinch of salt at the end.
Plop large spoonfuls of frosting equally across the cooled cake and spread an even layer of frosting using an flat icing spatula.