Make the buttercream while the cookies are cooling. You could also make this a day ahead of time and bring to room temperature before using, just whip again in your mixer to make it light and fluffy.
Set a pan of water over the stove and bring to a simmer. Set your mixing bowl over the pan to make sure it doesn’t touch the water, either pour out some of the water or add a foil ball to keep it from touching. In the stand mixer bowl, combine the egg whites, sugar and salt.
Set the bowl over the pan of simmering water until it reaches 160 deg f. ( use a candy thermometer). Return the bowl to the stand mixer and attach the whisk attachment. Slowly bring the speed all the way up and whip the mixture until thick and glossy and the bottom of the mixer bowl is cooler to the touch, about 7 minutes. Lower the speed to medium and add in the butter one tablespoon at a time allowing a few rotations of the whisk in-between each addition.
Stop to scrape down the sides of the bowl and bring the speed up to high and whip until the buttercream comes together. Mix the espresso powder with water then add to the buttercream. Whip to fully incorporate for another minute.
If piping the buttercream for the filling - fit a piping bag with a small open star tip and fill with some of the buttercream, squeeze to get rid of any air. I always squeeze a little back into the mixing bowl when getting rid of the air.