01 - Combine chopped dark chocolate and cubed butter in a heatproof bowl over simmering water. Stir continuously until smooth, then remove from heat and cool slightly.
02 - Whisk egg yolks with half of the sugar (2 tbsp) in a large bowl until pale and creamy.
03 - Gradually fold the melted chocolate mixture into the whisked egg yolks until fully incorporated.
04 - In a clean bowl, beat egg whites with a pinch of salt until soft peaks form. Slowly add remaining sugar (2 tbsp) and continue beating until glossy and stiff.
05 - Fold one-third of the egg whites gently into the chocolate mixture to lighten it, then carefully fold in the remaining egg whites in two additions without deflating.
06 - Whip chilled cream until soft peaks form and fold gently into the chocolate mixture for a smooth, uniform texture.
07 - Transfer the mousse into individual serving glasses or ramekins. Cover and refrigerate for at least 2 hours until set.
08 - Before serving, garnish with dark chocolate shavings, fresh berries, or whipped cream as desired.