Chop the dark chocolate into small pieces and place it in a heatproof bowl.

In a saucepan, heat 80 ml of the heavy cream with the espresso over medium heat until just simmering—do not boil.

In a separate bowl, whisk the egg yolks, sugar, vanilla, and salt together until pale and creamy.

Slowly pour the hot cream-espresso mixture into the egg mixture, whisking constantly to prevent scrambling.

Pour the combined yolk mixture back into the saucepan and cook over low heat, stirring constantly, until slightly thickened (about 3-4 minutes). Do not boil.

Pour the hot custard over the chopped chocolate. Let stand for 2 minutes, then stir until smooth and glossy.

Let the mixture cool to room temperature.

Whip the remaining 160 ml of heavy cream to soft peaks.

Gently fold the whipped cream into the cooled chocolate mixture in two additions, being careful not to deflate.

Spoon the mousse evenly into 4 ramekins or serving glasses.

Chill for at least 2 hours before serving.