In a blender or food processor, pulse the sugar until powdery. Transfer the sugar to a medium heatproof bowl and whisk in the egg whites and salt. Set the bowl over a saucepan of simmering water and whisk gently until the sugar is completely dissolved, 5 minutes.
Transfer the warm egg-white mixture to the bowl of a standing electric mixer fitted with the whisk. Add the vanilla and beat at medium-high speed until stiff and glossy, about 8 minutes. Beat in the butter a few pieces at a time, making sure it is fully incorporated before adding more. The buttercream should be light and fluffy; if it appears runny at any time, transfer the bowl to the freezer for 5 to 10 minutes, then return it to the mixer and continue.
Beat in the melted chocolate until fully incorporated, scraping down the side and bottom of the bowl. Remove the bowl from the mixer and beat with a wooden spoon to remove any air bubbles.