Place the chopped chocolate and butter in a heatproof bowl.
Heat 1/2 cup of the heavy cream until just steaming.
Pour the warm cream over the chocolate and let sit for 2 minutes.
Whisk until smooth and glossy.
Allow the chocolate mixture to cool to room temperature.
In a separate bowl, beat the remaining 1/2 cup heavy cream with powdered sugar and vanilla until soft peaks form.
Gently fold the whipped cream into the cooled chocolate mixture in three additions.
Mix carefully until no streaks remain.
Divide the mousse evenly among serving glasses.
Cover and refrigerate for at least 3 hours or until fully set.
Before serving, top with whipped cream, chocolate shavings, and fresh mint if desired.