In a large bowl, combine the softened butter and creamy peanut butter. Use an electric mixer to beat them until creamy and well-mixed, about 2-3 minutes.
Gradually add the powdered sugar, about 1/3 cup at a time, while mixing on low speed. Scrape the sides and bottom of the bowl to incorporate all ingredients thoroughly.
Stir in the vanilla extract and salt.
Keeping the mixer on low speed, pour in the milk and mix until everything is well combined. Then, increase the speed to high and beat for about 30 seconds until fluffy.
Spread or pipe the frosting onto completely cooled cupcakes or cake.
Notes
For a richer flavor, you can use salted butter instead of unsalted butter.