In a medium bowl, blend 1 cup of mascarpone cheese and 1/2 cup of powdered sugar using a hand mixer on low speed until the mixture is smooth and creamy, about 2-3 minutes.

In a separate bowl, whip 1 cup of heavy cream until stiff peaks form, about 3-4 minutes. Gently fold the whipped cream into the mascarpone mixture.

Carefully incorporate 1/2 cup of finely chopped pistachios into the mousse mixture.

Divide the mousse mixture into 6 serving glasses or ramekins. Cover with plastic wrap and refrigerate for at least 2 hours.

In a small saucepan, heat 3/4 cup of heavy cream until it just begins to simmer. Remove from heat and pour hot cream over 8 ounces of chopped chocolate, stirring until smooth.

Allow the chocolate ganache to cool at room temperature for about 10-15 minutes.

Just before serving, pour the cooled ganache over each glass of mousse.

Top each mousse with additional chopped pistachios or a dollop of whipped cream.