In a large mixing bowl, combine the 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. Add the vanilla, 2 tablespoons of cream, and salt. The frosting should be thick, creamy, and spreadable. If it becomes too thick to easily spread, add an additional tablespoon or two of cream 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.