Melt the dark chocolate with 250 milliliters of heavy whipping cream until completely smooth. Let it cool to room temperature.
In a bowl, whip the remaining 350 milliliters of heavy whipping cream with the powdered sugar (and vanilla bean) until stiff peaks form.
Gently fold one-third of the whipped cream into the chocolate ganache. Continue folding in the remaining whipped cream until the mousse is light and airy.
Transfer the mousse to a large bowl or individual cups and smooth the tops. Chill for 4 hours in the fridge.
RECIPE NOTES
For a softer mousse, remove it from the fridge 15 minutes before serving.