In a food processor, combine unsalted shelled pistachios and pulse for 1-2 minutes until finely ground.

In a mixing bowl, use an electric mixer to beat mascarpone until smooth and creamy, about 2 minutes. Gently fold in freshly whipped cream.

Add the ground pistachios and sugar to the mascarpone mixture and gently fold until uniformly mixed.

Divide the mousse mixture into serving glasses or bowls. Cover with plastic wrap and refrigerate for at least 4 hours.

In a small saucepan, heat heavy cream until it simmers. Pour it over finely chopped chocolate and stir until smooth.

Once the mousse has chilled, pour ganache over each serving and let cool slightly before serving.