In a large mixing bowl, use a hand mixer or whisk to whip sugar and egg yolks for 2-3 minutes until well combined and creamy.
In a medium-sized sauce pan, heat 1 cup of the whipping cream over medium heat until it reaches a simmer. Make sure to stir as it heats up to prevent burning. Once the cream has reached a simmer, work quickly and carefully pour half of the hot whipping cream in a steady stream into the egg mixture, whisking constantly to avoid curdling or cooking the eggs.
Immediately pour the egg mixture back into the saucepan, whisking constantly. Return to a medium heat and whisk constantly until mixture thickens, approximately 5 minutes.
Remove from heat and stir the chocolate into the hot mixture. Stir until all the chocolate has melted.
Cover and refrigerate for at least 2 hours.
In a chilled medium-sized mixing bowl, add in the 2 remaining cups of whipping cream and vanilla. Whip the mixture with a hand mixer until the cream has stiff peaks. Using a spatula, fold in the chocolate mixture into the whipped cream mixture and continue to gently fold until evenly mixed.
Serve with a dollop of whipped cream and chocolate shavings to garnish, if desired.
Notes
This is a very light mousse. If you would like to make it richer, only make 1 cup of the whipped cream to fold into the finished chocolate.