1. Combine the sugar, egg whites, and salt in the bowl of a stand mixer. Set 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 150°F on a candy thermometer or a digital thermometer, about 3 minutes.

2. Fit stand mixer with whisk attachment and beat the egg white mixture on medium speed until slightly cooled and thickened, about 2 minutes. Add the butter, 1 piece at a time, until smooth and creamy. If the mixture looks curdled, that’s okay. Just keep beating until it becomes smooth. This may take several minutes. If the butter becomes too warm and greasy, refrigerate the bowl for 10 to 15 minutes then continue to beat until smooth.

3. Add 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 of the bowl as necessary.

Make Ahead:

1. Frosting can be made 1 day ahead of time and stored in an airtight container in the refrigerator. Warm frosting in the microwave for 5 to 10 seconds and stir until creamy again before frosting cake.
2. Frosting can also be transferred to a ziptop bag and frozen for up to 1 month. Defrost in the fridge overnight then rewhip in the stand mixer until light and smooth.