01 - Combine dark chocolate and butter in a heatproof bowl set over barely simmering water, stirring until smooth. Remove from heat and allow to cool slightly.
02 - Whisk egg yolks with half of the sugar and vanilla extract until pale and creamy. Gradually incorporate the cooled chocolate mixture until fully blended.
03 - In a separate clean bowl, beat egg whites with a pinch of salt to soft peaks. Slowly add remaining sugar and continue beating until stiff, glossy peaks form.
04 - Whip cold heavy cream in another bowl until soft peaks develop.
05 - Gently fold the whipped cream into the chocolate mixture until just combined, then carefully fold in the beaten egg whites in three additions, preserving the airy texture.
06 - Divide mousse evenly among serving glasses or ramekins. Cover and refrigerate for at least 2 hours or until set.
07 - Sprinkle each portion with flaky sea salt and optionally decorate with dark chocolate curls or shavings prior to serving.