01 - Combine chopped dark chocolate and unsalted butter in a heatproof bowl set over simmering water. Stir continuously until melted and smooth. Remove from heat and let cool slightly.
02 - In a separate bowl, beat egg yolks with half the granulated sugar and vanilla extract until mixture becomes pale and creamy. Gradually fold in the melted chocolate mixture.
03 - In a clean bowl, whip egg whites with a pinch of salt until soft peaks form. Gradually add remaining granulated sugar and continue whipping until stiff peaks are achieved.
04 - Gently fold the beaten egg whites into the chocolate mixture in three additions, taking care to maintain airiness and avoid deflating.
05 - Portion the mousse evenly into four serving glasses. Cover and refrigerate for a minimum of two hours, or until firm.
06 - Using a chilled bowl, beat heavy cream with powdered sugar and vanilla extract until soft peaks form.
07 - Top each portion of mousse with a generous dollop of freshly whipped cream just before serving.