This silky chocolate mousse is a classic French dessert that delivers pure elegance in every spoonful. It's the perfect make-ahead treat for any special occasion.
Place the chopped chocolate and butter in a heatproof bowl set over a pot of barely simmering water. Stir occasionally until completely melted and smooth. Remove from heat and let the mixture cool for about 5 minutes until it is warm but not hot.
Stir the egg yolks into the warm chocolate mixture one at a time, mixing well after each addition until the mixture is thick and glossy. Set aside.
In a large clean bowl, pour in the cold heavy cream and vanilla extract. Using a hand mixer or whisk, beat until soft peaks form - the cream should hold its shape but still look smooth and billowy. Transfer to the refrigerator while you work on the next step.
In another clean bowl, beat the egg whites with the salt on medium speed until foamy. Gradually add the sugar while continuing to beat. Increase speed to high and beat until you reach stiff, glossy peaks that hold their shape when you lift the beaters.
Take one-third of the whipped egg whites and stir it into the chocolate mixture to lighten it. Then gently fold in the remaining egg whites in two additions, using a spatula to cut through the center and sweep along the bottom of the bowl. Fold just until no white streaks remain.
Gently fold in the whipped cream using the same technique, being careful to keep as much air in the mousse as possible. The mixture should be uniformly chocolatey with a light, airy texture.
Divide the mousse among 4 to 6 serving cups or ramekins. Cover each with plastic wrap and refrigerate for at least 4 hours or until fully set. Serve chilled with an extra dollop of whipped cream on top if desired.