01 - Combine dark chocolate and unsalted butter in a heatproof bowl set over simmering water. Stir until smooth, then remove from heat and allow to cool slightly.
02 - Whisk egg yolks with granulated sugar until pale and creamy. Incorporate vanilla extract, orange zest, and orange juice. Blend in melted chocolate mixture until homogeneous.
03 - In a clean bowl, beat egg whites with a pinch of salt until stiff peaks form.
04 - Whip cold heavy cream until soft peaks develop.
05 - Fold one-third of the whipped egg whites into the chocolate mixture to lighten it. Gently fold in the remaining egg whites in two additions, then fold in the whipped cream until fully integrated and smooth.
06 - Spoon or pipe the mousse into serving glasses or ramekins. Cover and refrigerate for at least 2 hours until set.
07 - Optionally garnish with additional orange zest, chocolate curls, or a dollop of whipped cream before serving.