In a mixing bowl, whip 1 cup of heavy whipping cream until soft peaks form, about 3–5 minutes.

In a separate bowl, combine 1 cup of mascarpone cheese, 1/2 cup of powdered sugar, and 1 teaspoon of vanilla extract. Mix until smooth.

Fold in 1/2 cup of finely chopped pistachios into the mascarpone mixture.

Gently fold the whipped cream into the pistachio mixture until uniformly combined.

Spoon the mousse into glasses and chill for at least 2 hours.

For the ganache, melt 6 ounces of dark chocolate with 3 tablespoons of unsalted butter in a heatproof bowl over simmering water, stirring until smooth.

Allow the ganache to cool slightly, then pour it over the set mousse.

Garnish with chopped pistachios or chocolate shavings before serving.