In a mixing bowl, whisk together the 2 eggs using an electric mixer on high speed for about 3 minutes until pale and fluffy. Gradually sprinkle in the 1/4 cup of sugar.

In a medium saucepan, heat 1 cup of heavy whipping cream over medium heat for about 5 minutes until just hot, stirring gently.

Carefully incorporate half of the hot cream into the whipped egg mixture to temper the eggs. Then pour it back into the saucepan and cook on low heat for about 5 minutes until slightly thickened.

Remove from heat and add in 6 ounces of dark chocolate, stirring continuously until melted and smooth. Allow the mixture to cool slightly, then refrigerate for about 2 hours to set.

In a chilled mixing bowl, whip 1 1/2 cups of heavy cream until stiff peaks form, about 3-5 minutes.

Gently fold the cooled chocolate mixture into the whipped cream, being careful to maintain the fluffy texture.

Spoon the mousse into individual cups or bowls, and top with shaved chocolate or fresh berries if desired. Serve immediately or refrigerate for up to 4 hours.