In a small bowl, mix boiling water and instant coffee until combined.
In a standing mixer or large bowl, cream butter for 3-5 minutes until very light and creamy.
Turn off the mixer and add powdered sugar. Beat on low until it starts to combine, then mix on medium for 3-4 minutes until light and fluffy.
Add the prepared coffee mixture and salt, then mix until combined.
Mix in milk or heavy whipping cream one tablespoon at a time until you reach your desired consistency.
Use this frosting on cakes, cupcakes, or cookies as desired.