In a mixing bowl, beat 1 cup of mascarpone cheese with an electric mixer on medium speed until smooth and creamy, about 2-3 minutes.

In a separate bowl, whip 1 cup of heavy cream until medium peaks form, around 3-4 minutes. Fold whipped cream into mascarpone mixture.

Finely chop ½ cup of pistachios and fold into the mousse gently.

Spoon the mousse mixture into serving glasses, cover, and refrigerate for at least 4 hours.

In a small saucepan, heat ½ cup of heavy cream until it begins to simmer. Add 4 oz of chocolate and stir until melted and smooth.

Once the mousse has set, pour or dollop the chocolate ganache over each mousse and serve.