Add egg whites, salt, and sugar in a bowl. Give the mixture a succinct whisk.
Put the bowl over a pan of simmering water. Be certain that the water doesn't touch the bowl. Whisk the egg whites sometimes while it warms up. After the mix has heated, whisk it continuously. You may heat the mixture until it reaches an internal temperature of 160ºF or tills it is not grainy between your palms.
Move the bowl into a rack mixer, combine with a whisk attachment until you obtain room temperature glistening peaks.
Switch into a paddle attachment and though the mix is operating on low, add tablespoon-sized dollops of room temperature butter which makes sure to allow the butter to incorporate until you add another piece. After all the butter has been incorporated, go right ahead and add the vanilla extract.
Drizzle in the melted and cooled chocolate while the mixer is operating on low. Scrape down the bowl and blend again. Now you can move it into a piping bag or into a ziplock bag to store in the freezer.