01 - Pulse the digestive biscuits in a food processor until finely ground, or place them in a sealed zip-top bag and crush with a rolling pin. Drizzle in the melted butter and pulse or stir until the crumbs are evenly moistened and cling together when pressed.
02 - Transfer the buttery crumb mixture into a 20 cm (8-inch) springform pan. Press firmly and evenly across the bottom and slightly up the sides using the back of a spoon or the flat base of a measuring cup. Refrigerate for 20 minutes to allow the base to firm up.
03 - In a large mixing bowl, combine the softened cream cheese, pistachio paste, sifted powdered sugar, and vanilla extract. Beat with an electric mixer on medium speed until completely smooth, scraping down the sides as needed.
04 - In a separate chilled bowl, whip the cold heavy cream with an electric mixer or whisk until stiff peaks form — the cream should hold its shape without sliding when the bowl is tilted.
05 - Gently fold the whipped cream into the pistachio-cream cheese mixture in two additions, using a large spatula and slow, sweeping motions. Take care to preserve as much volume and air as possible for a light, mousse-like texture.
06 - Spoon the filling over the chilled biscuit base, spreading it into an even layer. Smooth the surface with an offset spatula, working from the center outward to eliminate air pockets.
07 - Cover the pan tightly with plastic wrap and refrigerate for at least 4 hours, or ideally overnight, until the filling is firm and sliceable.
08 - Just before serving, scatter the chopped pistachios and any optional garnishes over the top. Run a thin knife around the edge of the pan, release the springform ring, slice into 8 portions, and serve chilled.