Chocolate Swiss Meringue Buttercream
5 large egg whites
1 cup granulated sugar
1 tsp salt
1 tsp vanilla extract
1 lb unsalted butter cut in to pieces, at room temperature
8 ounces bittersweet chocolate, melted and cooled
Place sugar, egg whites and salt in the heat-proof bowl of an electric mixer. Set bowl over a pan of gently simmering water, and whisk until sugar has dissolved and egg whites are hot to the touch, about 3 minutes. Test by rubbing the mixture between your fingers; it should feel completely smooth.
Add the butter, one piece at a time, and beat until incorporated after each addition. Don't worry if the buttercream appears curdled after all the butter has been added; it will become smooth again with continued beating. Add vanilla extract, and beat just until combined.
Add the chocolate and beat until it's fully incorporated in to the buttercream.