Melt chocolate. Place in microwave safe bowl and heat in 30-second increments. An alternative method would be to use a double broiler. Place chopped chocolate in heat proof bowl and set over saucepan of simmering water. Gently stir until chocolate is fully melted, then set aside to cool.
Place sugar in a medium heatproof bowl and add in the egg whites and salt.
Set the bowl over a saucepan of simmering water (about 2 inches deep) and whisk gently until the sugar is completely dissolved, about 5 minutes.
Transfer the warm egg-white mixture to standing mixer fitted with the whisk.
Add the vanilla and beat at medium-high speed until glossy and firm, about 8 minutes.
Beat in the butter a few pieces at a time, making sure it is fully incorporated before adding more.
If the buttercream 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.