01 - Combine dark chocolate and butter in a heatproof bowl over barely simmering water and stir until smooth. 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 incorporate the cooled chocolate mixture.
03 - Whisk egg whites with a pinch of salt until soft peaks form, then gradually add remaining sugar and continue beating until stiff and glossy peaks form.
04 - Whip the cold heavy cream until soft peaks form.
05 - Carefully fold whipped cream into the chocolate mixture until combined, then fold in the beaten egg whites in three additions, maintaining the mousse's light texture.
06 - Spoon mousse into serving glasses or ramekins, cover, and refrigerate for a minimum of 2 hours until set.
07 - Sprinkle each serving with flaky sea salt and optionally top with dark chocolate curls or shavings before serving.