In a large mixing bowl, add in the 1 cup butter and 1 1/2 cups creamy peanut butter. Using an electric mixer, beat on medium until combined and smooth.
Add in the 4 cups powdered sugar, 2 tablespoons of heavy whipping cream, and the 1 teaspoon vanilla extract. Beat on low until the powdered sugar is mostly incorporated.
If the frosting is thick, add more heavy cream one tablespoon at a time until you reach your desired consistency.
Press a piece of plastic wrap on top of the frosting so a crust doesn't form and store in the refrigerator.
Notes
Storage: Store covered or in an airtight container at room temperature for up to 3 days or in the refrigerator for up to 1 week.