Add the butter and peanut butter to a large mixing bowl.
Using an electric mixer (hand beaters or the paddle on a stand mixer), beat together on medium-high speed until smooth.
Add the powdered sugar, cocoa, milk, vanilla and salt, and beat together on low until the sugar and cocoa are mixed in. (If you set the mixer to high, powdered sugar and cocoa dust will go everywhere).
Increase the speed to medium-high and beat together until the frosting is smooth and creamy, stopping to scrape down the sides as needed.
Notes
The butter will need to be brought to room temperature in order to ensure a smooth and creamy buttercream. This may take an hour or longer depending on the temperature in your kitchen. To speed up the process, you can cut the butter into cubes.
Avoid using natural peanut butter. It will result in a runny and grainy frosting. Stick with shelf stable and smooth.
For the smoothest frosting, always sift your powdered sugar and cocoa to remove any lumps.
If you prefer a sweeter and thicker frosting, just add more powdered sugar, a little at a time, until it reaches your desired sweetness and thickness.
For a creamier frosting, just add a bit more milk (1/2 teaspoon at a time), until the frosting reaches your desired consistency.