Can be refrigerated for up to 3 days. Before using, bring to room temp.; beat with paddle attachment on med. speed until smooth.
Creamy Chocolate Buttercream Frosting
1 1/2 c. sugar
6 large egg whites
pinch of salt
1 lb. 6 oz. (5 1/2 sticks) unsalted butter, cut into TB. size pieces, softened
6 oz. semisweet chocolate, melted and cooled
Put sugar, egg whites, and salt in heatproof bowl of an electric mixer. Set over a pan of simmering water. Whisk constantly until sugar has dissolved and mixture registers 160 degrees on an instant read thermometer.
Return bowl to mixer, fit mixer with the whisk attachment. Beat on med. high speed until stiff, glossy peaks form, and mixture is cool, about 10 minutes.
Switch to the paddle attachment. With mixer on med. speed, add butter 1 or 2 TB. at a time, mixing well after each addition. Mix, scraping down sides of bowl as needed, until the mixture is smooth. Mix in cooled chocolate. Use immediately or cover and refrigerate.