In a large mixing bowl, using a hand mixer, whisk the sugar and egg yolks for about 3 minutes or until well combined, light, and creamy.
Place a medium saucepan, over medium heat, and add 1 cup of whipping cream. Heat until it starts to simmer and make sure you stir it all the time to avoid burning it.
Once the cream has reached a simmer, working VERY quickly pour 1/4 of the hot cream, in a steady stream into the egg mixture, while whisking CONSTANTLY to avoid curdling or cooking the eggs. Little by little, while whisking all the time, add the rest of the hot cream.
Once fully combined, immediately pour the egg mixture back into the saucepan, still whisking constantly. Return over medium heat and whisk constantly until it thickens, for about 4-5 minutes.
Remove from heat and immediately stir in the chopped chocolate until the chocolate has fully melted, and the mixture is shiny and smooth.
Cover with plastic wrap and refrigerate for at least 2 hours.
In a chilled medium-sized mixing bowl, add the remaining 2 cups of heavy whipping cream and the vanilla extract. Whip the mixture with a hand mixer until stiff peaks form.
Using a spatula, fold the whipped cream into the chilled chocolate mixture until well combined. Distribute into serving glasses and chill for at least 30 minutes before serving.
Serve the mousse topped with a dollop of whipped cream and chocolate shavings.