Place sugar and egg whites in the bowl of an electric mixer set over (but not touching) a pan of simmering water. Whisk constantly until mixture is heated and sugar is dissolved. Attach bowl to an electric mixer fitted with the whisk attachment and continue whisking until cooled and stiff peaks form, about 7 minutes.
With the mixer running, add butter, one piece at a time, until completely incorporated. Continue whisking until a light, fluffy cream has formed. Stir in chocolate until well combined.