01 - Combine dark chocolate and butter in a heatproof bowl over barely simmering water and stir until smooth. Remove from heat and cool slightly.
02 - Whisk egg yolks with half of the sugar and vanilla extract until pale and creamy. Slowly incorporate the cooled chocolate mixture until fully combined.
03 - Whip egg whites with a pinch of salt until soft peaks form. Gradually add remaining sugar, continuing to beat until stiff, glossy peaks develop.
04 - Whip cold heavy cream until soft peaks form.
05 - Gently fold whipped cream into the chocolate mixture until just combined, then fold in egg whites in three additions, taking care to preserve airiness.
06 - Spoon mousse into serving glasses or ramekins, cover, and refrigerate for at least 2 hours until set.
07 - Sprinkle each serving lightly with flaky sea salt and optionally garnish with chocolate curls or shavings before serving.