Finely chop chocolate; set aside. In a double boiler, whisk yolks, sugar, and salt until sugar has dissolved and mixture is warm to the touch, 2 to 3 minutes. Off heat, quickly whisk in chopped chocolate until melted and cocoa until smooth (mixture will be thick). Pour into a medium bowl and cool to room temperature.
In another medium bowl, whip heavy cream until soft peaks form. Whisk half of whipped cream into room-temperature chocolate mixture. Gently fold in remaining whipped cream with a rubber spatula. Divide mousse among four small serving dishes. Cover tops with plastic wrap. Chill at least 2 hours and up to 1 day.
Remove mousse from refrigerator 15 minutes before serving. Garnish with homemade whipped cream (recipe below) and shaved chocolate.
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