01 - Combine chopped dark chocolate and butter in a heatproof bowl over simmering water. Stir until smooth, then remove from heat and allow to cool slightly.
02 - Whisk egg yolks with half the sugar until pale and creamy. Incorporate peppermint extract and a pinch of salt.
03 - Gradually add the melted chocolate mixture into the egg yolk mixture, whisking to combine thoroughly.
04 - In a separate clean bowl, beat egg whites to soft peaks. Gradually add remaining sugar and continue whipping until stiff, glossy peaks form.
05 - Whip the cold heavy cream until soft peaks develop.
06 - Gently fold whipped cream into the chocolate mixture until just incorporated.
07 - Carefully fold beaten egg whites in two additions into the mixture, preserving as much air as possible to maintain lightness.
08 - Divide the mousse into serving glasses or ramekins. Cover and refrigerate for a minimum of 2 hours until set.
09 - Optionally top each serving with whipped cream, chocolate shavings, and crushed peppermint candies just before serving.