01 - Combine chopped dark chocolate and unsalted butter in a heatproof bowl set over simmering water. Stir until smooth, then remove from heat and allow to cool slightly.
02 - Whisk egg yolks with half of the granulated sugar and vanilla extract until pale and creamy, then gradually fold in the melted chocolate mixture.
03 - In a clean bowl, beat egg whites and salt until soft peaks form. Gradually add remaining sugar and continue beating until stiff peaks develop.
04 - Gently fold the beaten egg whites into the chocolate mixture in three additions, taking care not to deflate the mixture.
05 - Divide the mousse evenly among four serving glasses, cover, and refrigerate for at least 2 hours or until set.
06 - Whip the chilled heavy cream with powdered sugar and vanilla extract until soft peaks form.
07 - Top each mousse serving with a generous dollop of freshly whipped cream immediately before serving.