Finely chop the pistachios and set aside a small amount for decoration.

In a large mixing bowl, blend mascarpone cheese and sugar until smooth and creamy.

In a separate bowl, whip the heavy cream until soft peaks form, then fold it into the mascarpone mixture.

Gently fold in the chopped pistachios until evenly distributed.

Spoon or pipe the mousse into serving glasses and chill in the refrigerator for at least 2 hours.

Heat the heavy cream in a saucepan until just about to simmer. Pour over chopped chocolate and stir until smooth.

Once the mousse is set, pour the ganache over the top of each mousse glass.

Return to the refrigerator for another 30 minutes to allow the ganache to firm up.

Garnish with reserved chopped pistachios before serving.