1Combine the instant espresso/coffee with the whole milk and set aside.
2In the bowl of a stand mixer, beat the butter on high until creamy and smooth (about 5 minutes), scraping down the bowl and paddle a few times in between.
3Turn the mixer to low and add the powdered sugar a few cups at a time, scraping down the bowl and paddle between intervals.
4Add the vanilla, the coffee/espresso milk mixture, and salt.
5Mix on low until fully incorporated, about one minute.