01 - Combine the chopped dark chocolate and butter in a heatproof bowl set over a saucepan of gently simmering water. Stir continuously until the mixture is completely smooth and glossy. Remove from heat and allow to cool slightly for about 5 minutes.
02 - In a spotlessly clean bowl, beat the egg whites with a pinch of salt using an electric mixer on medium speed until soft peaks begin to form. Gradually add half of the granulated sugar, about one tablespoon at a time, and continue beating until stiff, glossy peaks hold their shape.
03 - In a separate bowl, vigorously whisk the egg yolks with the remaining sugar until the mixture turns pale yellow, thickens noticeably, and falls in ribbons from the whisk.
04 - Pour the slightly cooled chocolate mixture into the egg yolk mixture. Fold gently with a spatula until fully incorporated and no streaks remain.
05 - In a chilled bowl, whip the cold heavy cream with an electric mixer on medium-high speed until soft peaks form. Be careful not to overwhip.
06 - Gently fold the whipped cream into the chocolate-egg yolk base using a large spatula. Use sweeping, circular motions to maintain as much air in the mixture as possible.
07 - Add the stiff egg whites in three separate additions, folding each one in carefully with a spatula before adding the next. Work slowly and deliberately to avoid deflating the batter.
08 - Spoon or pipe the finished mousse into individual serving glasses or ramekins. Cover loosely with plastic wrap and refrigerate for a minimum of 2 hours, or until the mousse is fully set and chilled through.