01 - Place the chopped chocolate in a heatproof bowl over a pan of simmering water (double boiler). Stir until melted and smooth. Remove from heat and let cool slightly.
02 - In a large bowl, whisk egg yolks with half the sugar (0.75 oz) until pale and creamy.
03 - Stir the melted chocolate and vanilla extract into the egg yolk mixture until well combined.
04 - In a clean bowl, beat the egg whites and salt until soft peaks form. Gradually add the remaining sugar (0.75 oz) and beat until stiff peaks form.
05 - Gently fold the egg whites into the chocolate mixture in three additions, being careful not to deflate.
06 - In a separate bowl, whip 3/4 cup heavy cream to soft peaks. Gently fold into the chocolate mixture until no streaks remain.
07 - Divide the mousse evenly among four serving glasses. Cover and refrigerate for at least 2 hours, or until set.
08 - Just before serving, whip 2/3 cup heavy cream with powdered sugar and vanilla extract until soft peaks form.
09 - Top each mousse with a generous dollop of whipped cream and garnish with chocolate shavings or cocoa powder if desired.