In a large mixing bowl, whip the heavy cream using an electric mixer on medium speed until stiff peaks form, about 3 to 4 minutes.

Gently fold in the mascarpone until fully combined to create a smooth, creamy base.

Add the ground pistachios and vanilla extract to the mixture and fold gently for about 1 to 2 minutes.

Spoon the mousse into individual serving glasses and refrigerate for at least 4 hours.

To make the ganache, heat the heavy cream in a saucepan until steaming, then pour over chopped chocolate and stir until smooth.

Pour the ganache over the mousse cups before serving, and garnish if desired.