Place chopped chocolate in a heatproof bowl. Measure 1/2 cup of the cream into a small saucepan and keep the remaining cup refrigerated.
Heat the measured cream until steaming but not vigorously boiling. Pour over the chocolate and leave undisturbed for 3 minutes.
Stir slowly from the center until the chocolate melts into a glossy mixture. Add vanilla and salt, then cool 15–20 minutes until barely warm.
Whip the remaining cold cream in a separate chilled bowl to medium peaks. It should hold soft shapes without looking grainy or stiffly clumped.
Fold one-third of the whipped cream into the chocolate to lighten it. Gently fold in the remainder, stopping when no pale streaks remain.
Divide between six glasses, avoiding heavy tapping that would deflate the mousse. Smooth the tops lightly or leave soft spooned swirls.
Cover without touching the surface and refrigerate at least 3 hours. The mousse thickens as the chocolate cools and the cream structure settles.
Serve chilled with a few chocolate shavings if desired. Keep the remaining glasses refrigerated and use small portions because the mousse is rich.
