1/2 cup butter, softened
1 cup creamy peanut butter
3 tablespoons milk, or as needed
2 cups confectioners' sugar
Place the butter and peanut butter into a medium bowl, and beat with an electric mixer. Gradually mix in the sugar, and when it starts to get thick, incorporate milk one tablespoon at a time until all of the sugar is mixed in and the frosting is thick and spreadable. Beat for at least 3 minutes for it to get good and fluffy.