Remove the cream cheese, sour cream, and eggs from the refrigerator and allow them to come to room temperature for 1–2 hours. Line the base of a 23cm springform pan with parchment paper and lightly grease the sides.
Preheat the oven to 160°C (325°F). Pulse the biscuits and pistachios in a food processor until they resemble coarse crumbs. Mix with melted butter, sugar, and salt until evenly combined.
Press the crust mixture firmly into the bottom and slightly up the sides of the prepared pan. Bake for 10 minutes, then cool while keeping the oven on.
Beat the cream cheese on low speed until smooth. Add the sugar and pistachio paste and mix until fully incorporated.
Combine the sour cream and cornstarch in a small bowl until smooth. Add to the cream cheese mixture along with the almond extract and vanilla extract. Mix on low speed until combined.
Add the eggs two at a time, mixing gently after each addition just until incorporated. Scrape down the bowl and fold once with a spatula.
Place the springform pan inside a slightly larger cake pan, then place both inside a roasting pan. Pour the filling over the crust and smooth the surface.
Fill the roasting pan with boiling water until it reaches about two-thirds up the side of the inner pan, creating a water bath.
Bake for 70–75 minutes, until the edges are set and the center still has a gentle wobble.
Turn off the oven and prop the door open slightly. Leave the cheesecake inside for 1 hour. Remove from the water bath and cool on the counter for 30 minutes.
Transfer the cheesecake to the refrigerator and chill for at least 6 hours, preferably overnight.
To make the topping, place the chopped white chocolate in a heatproof bowl. Heat 100g heavy cream until steaming and pour over the chocolate in three additions, stirring until smooth.
Stir in the vanilla extract and the remaining cold heavy cream. Cover the surface and refrigerate overnight or until thoroughly chilled.
Whip the chilled ganache until soft peaks form. Spread or dollop over the chilled cheesecake.
Garnish with chopped pistachios. Slice with a clean sharp knife and serve cold.