Place chopped chocolate and butter in a heatproof bowl. Set the bowl over gently simmering water. Stir often until completely smooth. Remove it from the heat and let it cool slightly.

Place the egg yolks in a clean bowl. Whisk them gently with vanilla extract. Slowly add the warm chocolate mixture while whisking continuously. This keeps the yolks smooth and prevents them from cooking.

Pour cold heavy cream into a large mixing bowl. Add powdered sugar and beat until soft peaks form. Do not overbeat the cream. It should remain smooth, fluffy and easy to fold.

Add the egg whites and salt to another clean bowl. Beat until they become foamy. Slowly add the granulated sugar. Continue beating until soft and glossy peaks form.

Gently fold the whipped cream into the chocolate mixture. Use a rubber spatula and make slow movements. Avoid stirring too quickly. Keeping the mixture light helps create the classic mousse texture.

Fold the beaten egg whites into the chocolate mixture in two batches. Use gentle movements from the bottom upward. Stop mixing once everything looks evenly combined and fluffy.

Spoon the chocolate mousse into small serving glasses. Smooth the tops if desired. Cover each glass and refrigerate for at least four hours. Chilling gives the mousse its perfect creamy texture.

Remove the mousse from the refrigerator before serving. Add chocolate shavings or a small dollop of whipped cream. Serve immediately while the mousse is cold, creamy and beautifully set.