Begin by placing unsalted, shelled pistachios in a food processor. Pulse for about 1-2 minutes until they are finely ground.

In a mixing bowl, beat the mascarpone cheese with an electric mixer until smooth and creamy, about 2-3 minutes. Gently fold in freshly whipped cream until soft peaks form.

Incorporate the ground pistachios into the mascarpone and whipped cream mixture. Add sugar and mix until uniformly blended and a pale green color is achieved.

Spoon or pipe the mousse into serving glasses, about three-quarters full. Cover and refrigerate for at least 4 hours, or overnight.

In a saucepan, heat heavy cream until simmering. Pour over chopped chocolate and stir until smooth, creating a glossy ganache.

Once the mousse is set, pour a layer of ganache over each serving. Garnish with crushed pistachios or fresh mint sprigs.