Begin by placing 1 cup of mascarpone cheese in a medium mixing bowl. Add 1 cup of heavy whipped cream and gently fold together using a spatula, ensuring a creamy consistency. Incorporate ¾ cup of finely crushed pistachios and ½ cup of sugar until thoroughly combined and smooth.

In a small saucepan, heat 1 cup of dark chocolate and ½ cup of heavy cream over low heat, stirring continuously until the chocolate melts and the mixture becomes glossy, about 3-5 minutes. Remove from heat and allow the ganache to cool slightly.

Grab your serving glasses or bowls, and begin layering your dessert. Spoon approximately ½ cup of the pistachio mousse into each glass, smoothing the top with a spatula. Follow this with a generous drizzle of the warm chocolate ganache atop each mousse layer.

Cover the glasses with plastic wrap and place them in the refrigerator to chill for at least 2 hours.

Once fully chilled, remove the pistachio mousse from the refrigerator. Garnish each glass with whole pistachios or a dollop of additional whipped cream.