Prepare the Pistachio Cream:
In a food processor, grind pistachios until finely ground. Set aside a small handful of pistachios for garnish. In a large mixing bowl, combine mascarpone cheese, heavy cream, powdered sugar, vanilla extract, and pistachio paste (if using). Beat until smooth and creamy.

Soak the Ladyfingers:
Pour the cooled coffee into a shallow bowl. Optionally, add coffee liqueur to the coffee. Briefly dip the ladyfinger biscuits into the coffee, making sure they are soaked but not too soggy.

Assemble the Tiramisu:
In a trifle dish or a rectangular pan, layer the soaked ladyfingers at the bottom. Spread a generous layer of the pistachio mixture over the ladyfingers. Repeat the layers, finishing with a layer of pistachio cream on top.

Chill and Garnish:
Cover and refrigerate the tiramisu for at least 4 hours, or preferably overnight. Before serving, garnish with crushed pistachios for added texture and flavor.