Place the chopped chocolate in a heatproof bowl. Place the heatproof bowl over a medium saucepan with simmering water. Melt chocolate stirring frequently until smooth. Set aside to cool slightly.
Separate the eggs, placing the yolks in one small bowl and the whites in a grease-free bowl. Make sure there is no yolk in the whites, as this can prevent them from whipping properly.
Add the egg yolks into the slightly cooled melted chocolate and mix quickly and thoroughly until well combined and smooth. Scraping the sides of the bowl with a rubber spatula.
Using a hand mixer or a stand mixer with the whisk attachment, beat the egg whites on medium-high speed until they form soft peaks. Add sugar once you start to see bubbles while whipping to help stabilize the egg whites. Beat until stiff peaks form.
In another clean medium bowl with an electric mixer whip the heavy whipping cream until stiff peaks.
Take about one-third of the whipped egg whites and whisk them into the chocolate mixture to evenly disperse the chocolate and egg yolk mixture.
Carefully fold in the remaining egg whites continuing with folding it in one third at a time, ensuring not to deflate them. Use a gentle folding motion to mix until no streaks of egg whites remain.
Gently fold in the whipped cream half at a time until there are no more steaks.
Divide the mousse among individual glasses or bowls. Cover and refrigerate for at least 2 hours, or until set. Chilling allows the mousse to firm up.
Before serving garnish finished mousse with a dollop of whipped cream and a sprinkle of chocolate shavings.