Steps
1. Melt the dark chocolate in a heatproof bowl over a pot of simmering water, stirring until smooth.
2. In another bowl, whisk the egg yolk, sugar, and vanilla extract until pale and creamy.
3. Fold the melted chocolate into the egg mixture until combined.
4. In a separate bowl, whip the heavy cream with a pinch of salt until soft peaks form.
5. Gently fold the whipped cream into the chocolate mixture in three additions, being careful not to deflate the mousse.
6. Spoon the mousse into a serving glass and refrigerate for at least 1 hour before serving.