01 - Set up a double boiler by placing a heatproof bowl over simmering water. Add chopped bittersweet chocolate and stir continuously until completely smooth and melted. Remove from heat source and allow to cool for 5 minutes, stirring occasionally to prevent skin formation.
02 - In a clean mixing bowl, combine egg yolks with half of the granulated sugar (about 2 tablespoons) and vanilla extract. Beat vigorously with a whisk or electric mixer until the mixture becomes thick, pale yellow, and forms ribbons when the whisk is lifted.
03 - Pour the slightly cooled melted chocolate into the yolk mixture in a steady stream while whisking constantly. Continue whisking until fully incorporated and the mixture is uniform in color and texture. Set aside.
04 - In a completely clean and dry bowl, beat egg whites with a pinch of salt using an electric mixer on medium speed until foamy. Gradually sprinkle in the remaining granulated sugar while continuing to beat. Increase speed to high and continue beating until stiff, glossy peaks form when the beaters are lifted.
05 - In a chilled bowl, whip the cold heavy cream (200 ml) using an electric mixer or whisk. Beat until soft peaks form—the cream should hold its shape briefly when the beaters are lifted but the tips should curl over. Avoid overwhipping to prevent butter formation.
06 - Add one-third of the whipped cream to the chocolate-yolk mixture. Using a rubber spatula, gently fold in a downward motion, scraping the bottom and sides of the bowl, then lifting the mixture over the top. Repeat until the cream is just incorporated. Add the remaining whipped cream and continue folding until no white streaks remain.
07 - Add half of the beaten egg whites to the chocolate mixture and fold in gently using the same technique. Once partially combined, add the remaining egg whites and continue folding until the mixture is uniform, light, and airy. Take care not to deflate the mixture.
08 - Spoon or pipe the chocolate mousse into individual serving glasses, ramekins, or dessert bowls. Fill each vessel about three-quarters full. Cover tightly with plastic wrap and refrigerate for a minimum of 2 hours, or until the mousse is firm and set to the touch.
09 - Just before serving, combine the remaining heavy cream (150 ml) with powdered sugar and vanilla extract in a chilled bowl. Whip with an electric mixer or whisk until soft peaks form—the consistency should be light and spreadable but not stiff.
10 - Remove the chilled mousse from the refrigerator. Either pipe the whipped cream topping decoratively over each portion using a piping bag fitted with a star tip, or spoon it on in dollops. Serve immediately while the cream is fresh and the mousse is chilled.