Heat milk on stovetop or in a microwave until steaming but not bubbling. Add pistachio butter and stir well to mix. (Alternatively, you can add milk and pistachio butter to a milk frother cup to heat and froth at the same time, but please check your milk frother instructions and warrantee before attempting.)
In a 12 oz mug, pour your coffee or espresso. Add pistachio milk mixture, stir, and enjoy.
I use Pistakio brand pistachio spread, which has a strong pistachio flavor, no artificial preservatives, and is very affordable compared to other brands of pistachio butter. Flavor may differ if using a different brand.
For iced coffee, you can blend cold milk with pistachio butter or allow your heated pistachio milk to cool before pouring over ice and adding coffee. Remember the ice takes up a large amount of space, so you will need a 16 oz glass or larger.