In the bowl of a stand mixer, cream the butter on med-high until light and fluffy (about 5 minutes), scraping down the bowl and paddle a few times in between.
Add the powdered sugar a few cups at a time, scraping down the bowl and paddle between intervals.
Turn the mixer off and sift in the cocoa powder with a fine mesh sieve, then mix it in on low speed.
In a small bowl, combine the espresso powder (or instant coffee) with the whole milk before adding it in with the vanilla and salt.
Mix on low until fully incorporated and smooth, 1-2 minutes, scraping down the bowl and paddle as needed.
