Gently melt the chopped dark chocolate and butter in a saucepan over low heat, stirring until smooth. Remove from heat and let cool slightly.

In a mixing bowl, whisk the egg yolks with half of the granulated sugar until pale and creamy.

Gradually fold the slightly cooled chocolate mixture into the egg yolk mixture until well combined.

In a separate, clean bowl, whip the egg whites with a pinch of salt until soft peaks form, then gradually add the remaining sugar and continue whipping until stiff, glossy peaks form.

Gently fold about a third of the whipped egg whites into the chocolate mixture to lighten it, then carefully fold in the remaining egg whites until just combined, being careful not to deflate the mixture.

In another chilled bowl, whip the heavy cream with vanilla extract until medium peaks form.

Gently fold the whipped cream into the chocolate mixture until no streaks remain.

Pour the mousse into individual serving glasses or a large bowl, cover, and chill in the refrigerator for at least 2 hours, or until set.

Garnish with chocolate shavings or cocoa powder before serving.