Place the chopped chocolate and butter in a heatproof bowl.
Melt them gently over a double boiler, stirring until smooth.
Remove from the heat and allow the chocolate mixture to cool slightly.
In a separate bowl, beat the egg yolks with the vanilla extract.
Gradually mix the egg yolks into the slightly cooled chocolate mixture.
In another clean bowl, beat the egg whites with the salt until soft peaks form.
Gradually add the sugar and continue beating until the mixture becomes glossy and forms firm peaks.
In a separate bowl, whip the heavy cream until soft peaks form.
Gently fold the whipped cream into the chocolate mixture.
Carefully fold in the whipped egg whites in batches, being gentle to maintain the airy texture.
Divide the mousse among six dessert glasses or serving dishes.
Cover and refrigerate for at least 2 hours, or until set.
Garnish with chocolate shavings, whipped cream, or fresh berries before serving.