01 - Combine dark chocolate and butter in a heatproof bowl over barely simmering water. Stir until smooth. Remove from heat and allow to cool slightly.
02 - Whisk egg yolks with half the sugar and vanilla until pale and creamy. Gradually incorporate the cooled chocolate mixture until well blended.
03 - Whip egg whites with a pinch of salt until soft peaks form. Add remaining sugar gradually and continue beating until stiff, glossy peaks develop.
04 - Whip the cold heavy cream until soft peaks form.
05 - Fold the whipped cream gently into the chocolate base until just combined. Then fold in the egg whites in three additions, taking care not to deflate the mixture.
06 - Distribute mousse into serving glasses or ramekins. Cover and refrigerate for at least 2 hours until set.
07 - Sprinkle each serving with flaky sea salt and optionally garnish with dark chocolate curls or shavings.