01 - Combine chopped 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 the granulated sugar and vanilla extract until pale and creamy; gradually fold in the melted chocolate mixture.
03 - In a separate clean bowl, whisk egg whites and salt until soft peaks form; gradually add remaining sugar and continue beating to stiff peaks.
04 - Carefully fold the beaten egg whites into the chocolate mixture in three additions, preserving the airy texture.
05 - Distribute the mousse evenly among four serving glasses; cover and refrigerate for at least two hours until set.
06 - In a chilled bowl, beat 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 prior to serving.