If your pistachio butter feels too thick or firm to mix properly, place it into a small microwave-safe bowl and heat it for just a few seconds until it reaches a smooth and runny consistency.
In a small cup, measuring pitcher, or jar, combine the runny, cooled pistachio butter, heavy cream, milk, vanilla extract, and a small pinch of salt. If using an unsweetened pistachio butter, add 1 tsp maple syrup as well.
Submerge a handheld electric milk frother into the liquid and turn it on, blending continuously until the mixture thickens, expands, and turns into a luscious, pillowy foam. About 1 min.
Fill your favorite tall serving glass all the way to the top with fresh ice cubes.
Pour the cold brew directly over the ice, leaving about an inch or two of space at the top of the glass.
Slowly pour the freshly whipped pistachio cold foam over the surface of the cold brew and serve immediately. Enjoy!