7. Using the whisk attachment of a stand mixer, whip the butter on medium-high speed for 5 minutes, stopping once to scrape the sides of the bowl. Reduce the mixer speed to low and add the powdered sugar a little at a time, waiting until it is mostly incorporated before adding more.
Once all of the powdered sugar has been added, scrape the sides of the bowl and increase the speed to medium-high and whip until fluffy, about 1 to 2 minutes. Add the espresso and vanilla mixture and continue to mix at medium-high speed until it is completely incorporated, scraping the sides as necessary.