Mix hot water with the espresso powder (not instant espresso it needs to completely dissolve) and let sit to cool down to room temperature.

Place chopped up chocolate into a microwave safe bowl and heat in the microwave in 30 second increments stirring after every time until completely melted, let sit to cool down to room temperature. (you could also use a double boiler to melt the chocolate if you prefer)

Combine the egg whites and sugar in the heatproof bowl of a stand mixer set over a pot of simmering water as a double boiler.  Whisk the mixture constantly until the mixture looks white and frothy and all the sugar is dissolved.  This will take 2-4 minutes.  When you lift the whisk just "pinch" the stream of sugar and roll it between your fingers.  If you feel any granules of sugar you need to heat it longer.  It should feel completely smooth.

Place the bowl onto your stand mixer and using the whisk attachment start mixing and slowly raise speed up to medium high.  Beat the mixture until the bottom of the bowl is cool to the touch and stiff peaks form,  9-10 minutes.

Slow the mixer down to medium and start adding the butter 2-3 tbsp at a time making sure each new addition is mixed in before adding the next.  It may look like the frosting is separating a bit but it will come back together after a minute or two.

Add the chocolate mixture and the espresso mixture and beat to combine stopping to scrape down the bowl.  Switch to the paddle attachment and beat slow for approximately 5 minutes to get out all the air bubbles.