In a small bowl, combine the boiling water and instant coffee or espresso powder. Stir until fully dissolved for a smooth, rich coffee flavor.

In a standing mixer or large bowl, cream the room-temperature unsalted butter for 3-5 minutes until very light and airy.

Turn off the mixer and gently add the powdered sugar to the butter. Beat on low until it starts to combine.

Once it starts to come together, increase the speed to medium/medium-high. Mix for another 3-4 minutes until the frosting is light and fluffy.

Pour in the dissolved coffee mixture and salt, then mix until evenly combined.

Slowly mix in the milk or heavy whipping cream until you reach your desired consistency.