01 - Melt the chocolate in a heatproof bowl over a pan of simmering water (double boiler). Stir until smooth, then let cool slightly.
02 - Whisk the egg yolks with half the sugar and the vanilla extract until pale and creamy. Gently mix in the melted chocolate.
03 - In a clean bowl, whip the egg whites with the salt until soft peaks form. Gradually add the remaining sugar and beat until stiff, glossy peaks form.
04 - In another bowl, beat the heavy cream until soft peaks form.
05 - Gently fold the whipped cream into the chocolate mixture until just combined.
06 - Carefully fold in the egg whites in three additions, using a spatula, until the mousse is smooth and no white streaks remain.
07 - Spoon the mousse into serving glasses or bowls. Chill for at least 2 hours, or until set.
08 - For the whipped cream topping, whip the chilled heavy cream with the powdered sugar and vanilla extract until soft peaks form.
09 - Top the chilled mousse with whipped cream. Garnish with chocolate shavings or fresh berries, if desired. Serve immediately.