01 - Set a heatproof bowl over a saucepan of gently simmering water to create a bain-marie. Add the chopped dark chocolate and butter, stirring occasionally until completely smooth. Remove from heat and let cool slightly for about 5 minutes.
02 - In a clean, grease-free 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 sugar, about 1 tablespoon at a time, then increase speed to high and continue beating until stiff, glossy peaks hold their shape.
03 - In a separate bowl, whisk the egg yolks with the remaining sugar vigorously until the mixture turns pale yellow and falls in thick ribbons from the whisk, about 2 to 3 minutes.
04 - Pour the slightly cooled chocolate-butter mixture into the egg yolk mixture. Fold gently with a spatula until fully incorporated and no streaks of yolk 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; the cream should hold its shape but still appear smooth and silky.
06 - Add the whipped cream to the chocolate-egg yolk mixture in two additions. Fold gently with a spatula using wide, sweeping motions to preserve as much air as possible while achieving a uniform color.
07 - Fold the stiff egg whites into the chocolate mixture in three separate additions. Use the lightest folding motion possible, scooping from the bottom and turning over the top. A few white streaks remaining after the final addition are acceptable to keep the mousse airy.
08 - Divide the mousse evenly among 4 individual serving glasses or ramekins. Smooth the tops lightly with a spatula. Cover each with plastic wrap and refrigerate for at least 2 hours, or until fully set and firm to the touch.