Step 1

In a small saucepan, bring a small amount of water to a boil. Reduce the heat to simmer.

Step 2

In medium heat-proof bowl (I used my stand mixer bowl), whisk together egg whites and sugar and place it over the saucepan with simmering water. (This is a make-shift double boiler. Make sure the bottom of the top bowl doesn't touch the water.)

Step 3

Cook the egg white mixture until sugar is completely melted and it reaches 160°F (70°C) stirring continuously, about 5 minutes. (This's my favorite thermometer!)

Step 4

Now, remove the bowl with egg white mixture from heat. Then whisk it on medium speed for a minute, then slowly increase the speed to medium high and beat until stiff peaks form and the mixture is cool to touch, about 10 minutes. (It may take longer with a hand-mixer.)

Step 5

Meanwhile, melt the chocolate in a microwave according to package directions. Let it cool.

Step 6

Once the meringue reached stiff peaks, scrape the side of the bowl with a spatula and add salt.

Step 7

Turn on the mixer on medium speed and start adding butter, one tablespoon at a time. Make sure the butter is fully incorporated before adding the next piece. (NOTE: It's normal if buttercream starts to curlde half way. Keep adding the butter!)

Step 8

Keep whisking the buttercream until smooth, light and fluffy, about 10 minutes.

Step 9

Now, switch to a paddle attachment and add cooled melted chocolate. Beat the buttercream until well combined, about a minute.