01 - Combine chocolate and butter in a heatproof bowl over simmering water and stir until smooth, then remove from heat and allow to cool slightly.
02 - Whisk egg yolks with half the sugar and vanilla extract until pale and creamy, then gradually fold in the melted chocolate mixture.
03 - In a clean bowl, whisk egg whites with salt until soft peaks form. Slowly add remaining sugar and continue to beat until stiff peaks form.
04 - Gently fold the beaten egg whites into the chocolate mixture in three additions, taking care not to deflate the mousse.
05 - Spoon mousse into 4 serving glasses, cover, and refrigerate for a minimum of 2 hours until set.
06 - Beat chilled heavy cream with powdered sugar and vanilla extract until soft peaks form.
07 - Top each portion of mousse with a dollop of freshly whipped cream immediately before serving.