In a small food processor, pulse pistachios and sugar until finely ground. With the motor running, stream in milk until a smooth pistachio paste forms.

In a mixing bowl, whip heavy cream with powdered sugar and vanilla to stiff peaks. Gently fold in mascarpone until fully incorporated.

Reserve ¼ cup of pistachio paste for topping. Swirl the remaining into the mascarpone mixture until pale green and marbled.

In a shallow dish, combine cooled coffee and pistachio liqueur (if using). Quickly dip each ladyfinger just to moisten (do not soak).

Arrange half the soaked ladyfingers in an even layer in a 9×9‑inch dish. Spread half the creamy mixture over them.

Repeat layering with remaining ladyfingers and cream. Smooth the top.

Drizzle reserved pistachio paste over the surface. Chill for at least 4 hours (or overnight) to set.

Just before serving, dust with cocoa or matcha powder and sprinkle with chopped pistachios.