1. In a small bowl, dissolve espresso powder in hot water; set aside. In the heatproof bowl of an electric mixer, combine egg whites, sugar, and salt. Set the bowl over a pan of simmering water. Whisk constantly until sugar has dissolved and whites are hot to the touch, 3 to 5 minutes.

2. Transfer mixture to the bowl of an electric mixer fitted with the whisk attachment and mix on low speed, gradually increasing to high speed, until stiff, glossy peaks form, about 10 minutes.

3. With machine running on low speed, add butter to the egg whites, beating until smooth. Add the espresso mixture and continue beating until incorporated.