Beat the cold heavy cream with the vanilla using an electric mixer until stiff peaks form, then cover and chill.
In a large metal bowl whisk together the egg yolks, room-temperature butter, cooled espresso, 2 tablespoons of the sugar, and sea salt.
Set the bowl over a saucepan of gently simmering water (bowl should not touch the water) and whisk constantly until the mixture lightens in color and almost doubles in volume; cook until an instant-read thermometer reads 160°F, about 1–5 minutes.
Remove the bowl from the heat and immediately whisk in the chopped chocolate until fully melted and smooth. Let the mixture cool to room temperature, whisking occasionally.
In a separate medium bowl, beat the egg whites with an electric mixer on medium speed until foamy. With the mixer running, gradually add the remaining 1 tablespoon sugar, then increase speed to high and beat until firm peaks form.
Gently fold the beaten egg whites into the cooled chocolate mixture in two additions, then fold in the chilled whipped cream until just combined and airy.
Divide the mousse among six 6–8 ounce ramekins and chill until firm, at least 2 hours.
Just before serving, top each mousse with a dollop of sweetened whipped cream, chocolate shavings, and fresh berries; let stand 10 minutes at room temperature if desired.