01 - Combine chopped dark chocolate and unsalted butter in a heatproof bowl over gently simmering water. Stir continuously until smooth. Remove from heat and allow to cool slightly.
02 - Whisk egg yolks with half the granulated sugar and vanilla extract in a clean bowl until thickened and pale.
03 - Fold the melted chocolate mixture gently into the egg yolk mixture until fully blended.
04 - In a separate clean bowl, beat egg whites with a pinch of salt until soft peaks form. Gradually add the remaining sugar and continue beating to glossy, stiff peaks.
05 - Fold one-third of the beaten egg whites into the chocolate mixture to lighten it. Carefully fold in the remaining egg whites until incorporated without deflating.
06 - Whip cold heavy cream to soft peaks in a separate bowl. Gently fold the cream into the chocolate mixture until homogenous and free of streaks.
07 - Distribute mousse evenly into four serving glasses or ramekins. Cover and refrigerate for a minimum of 2 hours until fully set.
08 - Optionally adorn with shaved chocolate, fresh berries, or additional whipped cream before serving.