⅔ cup (116g) sugar
4 (140g) large egg whites
¼ teaspoon salt
1½ cups (339g) unsalted butter, cut into pieces, softened
284g semi-sweet chocolate, melted and cooled
1 teaspoon vanilla extract
Mix the sugar, egg whites and salt in the bowl of a stand mixer. Place the bowl over a saucepan filled with an inch of barely simmering water. Whisk gently and continuously while heating the icing. It should read 65°C on an instant-read thermometer. The sugar should be completely dissolved.
Transfer the bowl to the stand mixer and secure with the whisk attachment. Beat on medium speed until the mixture has the consistency of shaving cream and cools slightly, 1 to 2 minutes. (If the meringue is hot, it will melt the butter).
Gradually add the butter, 1 tablespoon at a time, until smooth and creamy (only add the next tablespoon of butter when the previous one has been fully mixed in).
Once all the butter has been added, scrape down the sides of the bowl and mix on medium-high speed and beat until light and fluffy, about 30 seconds.
Add the cooled melted chocolate and vanilla. Mix on medium speed until well blended. Increase speed to medium-high and beat until light and fluffy, about 30 seconds.