Melt the chopped chocolate in a heatproof bowl set over simmering water, stirring until smooth, then remove from heat and let cool for 5 minutes.
Whisk the egg yolks into the slightly cooled chocolate until well combined and smooth.
In a separate clean bowl, beat the egg whites with an electric mixer until soft peaks form, then gradually add the sugar and continue beating until stiff, glossy peaks form.
In another bowl, whip the heavy cream with vanilla extract until soft peaks form.
Gently fold the whipped cream into the chocolate mixture, then carefully fold in the beaten egg whites until no white streaks remain.
Divide the mousse among 6 serving glasses or bowls, cover, and refrigerate for at least 2 hours or until set before serving.