Melt the dark chocolate in a heatproof bowl over simmering water until smooth.

In a separate bowl, whip ¾ cup of heavy cream until soft peaks form.

Whisk egg yolks, sugar, and vanilla extract together until pale and creamy.

Once the chocolate cools slightly, gently fold it into the egg yolk mixture.

Carefully fold in half of the whipped cream until combined, then add the remaining whipped cream until no white streaks remain.

Spoon the mousse into serving dishes and chill in the refrigerator for at least two hours before serving.