Melt chocolate in a heatproof bowl set over a pan of simmering water, stirring. Remove from heat, and keep warm.
Put egg whites and sugar into the heatproof bowl of an electric mixer set over a pan of simmering water. Whisk until sugar has dissolved and mixture reaches 110 degrees. Attach bowl to mixer fitted with the whisk attachment. Beat on high speed until fluffy and completely cool.
Switch to the paddle attachment. On medium-low speed, add butter, 2 pieces at a time, beating until combined after each addition. Beat in chocolate. Raise speed to high, and beat until smooth, about 5 minutes. Beat on low speed to eliminate air bubbles. Buttercream can be refrigerated in an airtight container up to 3 days or frozen up to 1 month; bring to room temperature, and beat on low speed until smooth.