- 10 large egg whites
- 2 1/2 cups sugar
- 32 ounces (8 sticks) unsalted butter, softened
- 24 ounces semisweet chocolate, melted
Whisk whites and sugar in a heatproof mixer bowl until combined. Set bowl over a large saucepan of simmering water, whisking until sugar dissolves and mixture registers 160 degrees on a candy thermometer, about 5 minutes.
Transfer bowl to mixer, and whisk on high speed until stiff, glossy peaks form and mixture is cool, about 10 minutes. Reduce speed to medium, and add butter, 1 tablespoon at a time, whisking until incorporated before adding more. Reduce speed to low, and whisk in chocolate. Raise speed to medium-high, and whisk until well combined. (Buttercream can be refrigerated up to 3 days or frozen for up to 1 month. Bring to room temperature before using; beat on low speed until smooth.)