In a large mixing bowl, combine the cream cheese, butter, and peanut butter. Beat with an electric mixer (with the wire whisk attachment for stand mixers) until thoroughly combined.
Gradually add the powdered sugar and beat until fluffy. The frosting should be thick, creamy, and spreadable. If it becomes too thick to easily spread, add a tablespoon of cream or milk and beat again, adding additional liquid only if necessary.
Notes
This recipe will make enough frosting for a two-layer cake. For a pan of brownies or a 9x13 cake, you will want to halve the recipe.