01 - Place the chopped chocolate in a heatproof bowl over a pan of simmering water, creating a double boiler. Stir continuously until completely melted and smooth. Remove from heat and allow to cool slightly while preparing the egg mixture.
02 - In a large mixing bowl, whisk the egg yolks with half of the granulated sugar (approximately 0.7 oz) until the mixture becomes pale, thick, and creamy in texture.
03 - Stir the melted chocolate and vanilla extract into the prepared egg yolk mixture, folding gently until completely incorporated and smooth.
04 - Using a clean, dry bowl and electric mixer or whisk, beat the egg whites with salt until soft peaks form. Gradually add the remaining granulated sugar and continue beating until stiff, glossy peaks form.
05 - Gently fold the whipped egg whites into the chocolate base in three separate additions, using a spatula with light, lifting motions to preserve the airy texture without deflating the mixture.
06 - In a separate chilled bowl, whip 0.85 cup cold heavy cream until soft peaks form. Carefully fold the whipped cream into the chocolate mixture until fully incorporated with no visible streaks remaining.
07 - Evenly distribute the chocolate mousse among four individual serving glasses. Cover with plastic wrap and refrigerate for at least 2 hours, or until completely set and firm.
08 - Just before serving, whip 0.63 cup cold heavy cream with powdered sugar and vanilla extract until soft peaks form, being careful not to overbeat.
09 - Top each chilled mousse with a generous dollop of freshly whipped cream. Garnish with chocolate shavings or a dusting of cocoa powder if desired, and serve immediately.