01 - Pulse the digestive biscuits in a food processor until fine crumbs form. Alternatively, place them in a sealed zip-top bag and crush thoroughly with a rolling pin.
02 - Pour the melted butter over the biscuit crumbs and stir until the mixture resembles wet sand and holds together when pressed.
03 - Transfer the crumb mixture into an 8-inch springform pan. Press it firmly and evenly across the bottom using the back of a spoon or the base of a measuring cup. Refrigerate for 20 minutes to allow the base to firm up.
04 - In a large mixing bowl, combine the softened cream cheese, pistachio paste, powdered sugar, and vanilla extract. Beat with an electric mixer on medium speed until the mixture is completely smooth and free of lumps.
05 - In a separate chilled bowl, whip the cold heavy cream with an electric mixer or whisk until stiff peaks hold their shape when the beaters are lifted.
06 - Gently fold the whipped cream into the pistachio-cream cheese mixture using a spatula, working in three additions. Use slow, sweeping motions to preserve as much volume and airiness as possible.
07 - Pour the filling over the chilled biscuit base and spread it into an even layer using an offset spatula, smoothing the surface.
08 - Cover the pan tightly with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, until the cheesecake is fully set and firm to the touch.
09 - Just before serving, sprinkle the chopped pistachios and any optional toppings over the surface. Run a thin knife around the edge of the pan, release the springform ring, slice into wedges, and serve chilled.