Start by finely chopping the dark chocolate bar. Place the chopped chocolate in a heatproof bowl. You can melt it using a double boiler over simmering water or in the microwave in 20-second bursts, stirring in between until smooth. Set aside to cool slightly.
In a mixing bowl, add the 4 egg yolks and ½ cup of sugar. Using a whisk or electric mixer, beat the mixture until it becomes pale and slightly thickened. This will take about 3-5 minutes.
Once the chocolate has cooled slightly, stir in the Grand Marnier, then slowly add the chocolate to the egg yolk mixture. Mix until fully combined and smooth.
In a separate clean bowl, beat the 4 egg whites until stiff peaks form. This step is crucial for achieving the light and airy texture of your mousse.
Gently fold the whipped egg whites into the chocolate mixture, being careful not to deflate the air you've just whipped into the egg whites. Use a rubber spatula and fold in thirds, ensuring everything is well incorporated.
Finally, gently fold in the crème fraîche until the mixture is uniform and creamy.
Spoon the mousse into individual serving dishes or a large bowl. Refrigerate for at least 2 hours to set before serving. Garnish with additional crème fraîche or chocolate shavings, if desired.
