In a large mixing bowl using a handheld mixer, beat the butter and peanut butter until smooth and creamy, stopping to scrape down sides as needed. (approx. 1-2 minutes)
Add the powdered sugar and mix on low speed at first to incorporate then increase to medium speed and continue mixing for 1-2 minutes or until fully combined.
Add the heavy cream (or milk), vanilla extract, and salt and continue mixing on medium speed until everything is well-combined, smooth, and easy to pipe.
Notes
Nutrition Facts below are estimated using an online recipe nutrition calculator. Values provided are per piped swirl of frosting if making 24 cupcakes. Adjust as needed if frosting 12 cupcakes with tall frosting or making a cake/cookie cake.