Chocolate Swiss Meringue Buttercream

Put sugar and egg whites in the heatproof bowl of an electric mixer. Set bowl over a pan of simmering water; whisk until sugar is dissolved, about 5 minutes.

Fit mixer with the whisk attachment. Beat the egg-white mixture on medium speed until stiff peaks form and mixture is completely cooled, about 10 minutes.

Beat in butter a few tablespoons at a time. Reduce speed to low; add vanilla and melted chocolate. Beat until mixture is smooth, about 5 minutes.