01 - Set up a double boiler by placing a heatproof bowl over a saucepan of simmering water. Add the chopped chocolate and stir constantly until completely melted and smooth. Remove from heat and allow to cool for 5 minutes.
02 - In a clean mixing bowl, combine egg yolks with half of the granulated sugar (about 2 tbsp) and vanilla extract. Beat vigorously with an electric mixer or whisk until the mixture becomes thick, pale, and slightly ribbony.
03 - Pour the cooled melted chocolate into the yolk mixture in a slow, steady stream while whisking continuously. Fold gently until fully incorporated and no streaks remain.
04 - In a separate grease-free bowl, beat the egg whites with a pinch of salt until foamy. Gradually add the remaining sugar and continue beating until stiff, glossy peaks form when the beaters are lifted.
05 - Using chilled beaters and a cold bowl, whip 3/4 cup heavy cream until soft peaks form—the cream should hold its shape briefly when the beaters are lifted.
06 - Add one-third of the whipped cream to the chocolate-yolk base and fold gently using a spatula. Add the remaining cream and fold until just combined, maintaining as much air as possible.
07 - Gently fold the beaten egg whites into the chocolate mixture in two additions. Use a light hand with sweeping motions from the bottom to preserve the airy texture.
08 - Divide the mousse evenly among serving glasses or bowls. Cover each container with plastic wrap and refrigerate for at least 2 hours until completely set and firm.
09 - Just before serving, whip 2/3 cup heavy cream with powdered sugar and 1/2 tsp vanilla extract until soft peaks form. The cream should be light and spreadable but not over-whipped.
10 - Pipe or spoon the sweetened whipped cream over each portion of chilled chocolate mousse. Serve immediately or return to the refrigerator for up to 1 hour before serving.