To make the frosting: In the bowl of an electric mixer fitted with the paddle attachment, beat together butter, cream cheese, and peanut butter on medium speed until smooth and creamy, about 2 minutes. Add 1 cup of powdered sugar and beat until smooth. Beat in vanilla and salt until smooth and fluffy. Add the rest of the powdered sugar as needed until desired consistency (thick enough to pipe). Frost cupcakes once they’ve cooled.
Because of the cream cheese frosting, cupcakes should be stored (covered, in an airtight container) in the fridge. They’ll keep for 3 days. Bring to room temperature about 1 hour before serving.