Heat 1/2 cup of heavy cream in a saucepan until hot but not boiling.
Pour over the chopped chocolate and let sit for 1–2 minutes, then stir until smooth.
Let cool to room temperature.
Whip remaining 1/4 cup of cream to soft peaks and fold into the cooled chocolate mixture with vanilla.
Pipe or spoon mousse over the chilled cheesecakes.
Chill until ready to serve.