01 - Set up a double boiler by placing a heatproof bowl over a pan of simmering water. Add the chopped chocolate and stir constantly until completely smooth and melted. Remove from heat and allow to cool for 5 minutes until lukewarm.
02 - In a mixing bowl, whisk together the egg yolks with 1 tablespoon of granulated sugar and 1 teaspoon vanilla extract. Beat vigorously until the mixture becomes pale yellow and slightly thickened, approximately 2-3 minutes. Gradually pour in the melted chocolate while whisking continuously to incorporate fully.
03 - Using a completely clean bowl and beaters, whip the egg whites with a pinch of salt on medium speed until soft peaks form. Slowly sprinkle in the remaining 1 tablespoon of sugar while continuing to beat. Increase speed to high and whip until stiff, glossy peaks form when the beaters are lifted.
04 - In a separate chilled bowl, beat the 3/4 cup heavy cream on medium-high speed until soft peaks form. The cream should hold its shape but still have a slight curl when the beaters are lifted.
05 - Add one-third of the whipped cream to the chocolate mixture. Using a rubber spatula, fold gently in a circular motion from the bottom up, rotating the bowl as you go. Repeat with remaining cream until just incorporated—some small white streaks are acceptable.
06 - Add the whipped egg whites in three separate additions. Fold the first third gently until partially blended, then repeat with second third. For the final addition, fold carefully until the mousse appears uniform with no visible white streaks remaining. Take care not to deflate the mixture.
07 - Spoon or pipe the mousse into individual serving glasses or bowls, filling each about three-quarters full. Smooth the tops with the back of a spoon. Refrigerate uncovered for at least 2 hours, or until completely set and firm to the touch.
08 - Just before serving, whip the chilled 2/3 cup heavy cream with powdered sugar and 1/2 teaspoon vanilla extract on medium-high speed. Beat until soft peaks form—the cream should hold its shape but still appear lush and creamy, not over-whipped.
09 - Remove the chilled mousse from the refrigerator. Top each serving with a generous dollop of whipped cream. Garnish with chocolate shavings or fresh berries if desired. Serve immediately while the mousse is firm and cold.