• Combine the sugar, egg whites, and salt in the bowl of a stand mixer. Set the bowl over a saucepan filled with 1-inch of gently simmering water. Whisking gently and constantly, heat mixture until sugar is dissolved and mixture is slightly thickened and foamy and registers 160°F on a candy thermometer or a digital thermometer, about 3 minutes. Remove from heat and carefully wipe the bottom of the bowl dry.
• Attach the bowl to the mixer fitted with the whisk attachment and begin to whip, until the meringue is thick, glossy, and stiff peaks are achieved. The bottom of the bowl must no longer feel warm to the touch. This should take about 7 to 10 minutes.
• Switch over to paddle attachment and, with mixer on low speed, add the butter cubes, one at a time, until incorporated. Continue beating until it has reached a silky smooth texture. This may take several minutes.
• If the buttercream curdles, simply keep mixing and it will come back together and be smooth. If the buttercream is too thin and runny, refrigerate for about 15 minutes before continuing mixing with the paddle attachment until it comes together.
• Add cooled chocolate and vanilla and mix until combined. Increase speed to medium-high and beat until light, fluffy, and well combined, about 30 seconds, scraping down the sides and bottom of the bowl as necessary.

Make Ahead:

• Keep in an airtight container in the refrigerator for up to 5 days or in the freezer for up to 2 months**. Let come to room temperature and beat on low with the paddle attachment, until creamy, before using. If frosting appears runny or curdled, microwave about 1/4 cup for 5-10 seconds, then mix back into bowl and beat on medium-high until smooth.