01 - Melt the dark chocolate and butter together in a heatproof bowl set over a pot of barely simmering water, stirring until smooth. Remove from heat and let cool slightly.
02 - Whisk the egg yolks with half the sugar and vanilla extract until pale and creamy. Slowly whisk in the slightly cooled melted chocolate mixture until fully combined.
03 - Beat the egg whites with a pinch of salt until soft peaks form. Gradually add the remaining sugar and continue beating until stiff, glossy peaks form.
04 - In another bowl, whip the cold heavy cream until soft peaks form.
05 - Gently fold the whipped cream into the chocolate mixture until just combined. Then fold in the beaten egg whites in three additions, being careful not to deflate the mixture.
06 - Spoon the mousse into serving glasses or ramekins. Cover and chill in the refrigerator for at least 2 hours or until set.
07 - Before serving, sprinkle each mousse with a pinch of flaky sea salt and garnish with chocolate curls or shavings if desired.