1. Place chocolate, butter, and 1/4 cup water in a large heatproof bowl set over (but not touching) simmering water; cook, stirring until chocolate and butter have melted and mixture is well combined. Remove from heat and let cool slightly.

2. In a large copper bowl, or the bowl of an electric mixer, whisk together egg yolks and 4 teaspoons sugar until pale yellow in color. Add some of the melted chocolate mixture to egg yolk mixture; whisk until well combined. Continue to slowly add chocolate mixture to egg yolk mixture, whisking constantly, until completely combined.

3. In a large copper bowl, whisk egg whites and salt until soft peaks form. Add remaining 4 teaspoons sugar and whisk until semifirm peaks form, 10 to 12 minutes. Gently fold egg white mixture into egg yolk mixture.

4. Gently transfer mousse to a 2-quart ceramic dish and transfer to refrigerator. Refrigerate at least 8 hours and up to overnight before serving. Serve chilled garnished with whipped cream, if desired.