Whip 1 cup of heavy cream with an electric mixer until soft peaks form. Refrigerate while preparing the rest.
In a heatproof bowl, whisk together egg, egg yolks, sugar, espresso powder, vanilla, and salt. Place over simmering water (double boiler) and whisk for 4-6 minutes until pale and thick.
In another heatproof bowl, melt the chopped bittersweet and semisweet chocolate over the same double boiler, stirring until smooth. Let it cool slightly.
Slowly whisk the warm egg mixture into the melted chocolate until combined.
Gently fold in the whipped cream in three batches, using a rubber spatula to maintain the airy texture.
Divide the mousse into four ramekins or dessert glasses and refrigerate for at least 1 hour before serving.
Garnish with whipped cream, chocolate shavings, or fresh berries before serving. Enjoy!
Notes
For a richer taste, use high-quality chocolate with at least 60% cocoa content. Chill for at least an hour for the best texture!