1. Combine egg whites and sugar in the heatproof bowl of a standing mixer; set over a saucepan of simmering water, whisking (by hand) until sugar dissolves and mixture registers 160 degrees on a candy thermometer, about 5 minutes.

2. Transfer bowl to mixer, and whisk on high speed until stiff, glossy peaks form and mixture is cool, about 10 minutes. 3. Reduce speed to medium, and add butter, 1 tablespoon at a time, whisking until combined before adding more. Reduce speed to low; whisk in chocolate, then whisk over medium-high until well combined. Buttercream can be refrigerated up to 3 days, or frozen up to 1 month, in an airtight container. Before using, bring to room temperature and beat on low speed until smooth.