1. Prepare the crust. Pulse pistachios in a food processor until finely chopped. Add flour and sugar, and pulse until the mixture is mealy. In a small bowl, whisk melted butter, egg yolk, and vanilla. Add to the processor and pulse until the mixture holds together when pinched.
2. Press the mixture into the bottom and about half an inch up the sides of a nine-inch springform pan or a disposable deep nine-inch round pan. Refrigerate for 20 minutes.
3. Preheat oven to 325 degrees Fahrenheit. Bake the crust until lightly browned, 10 to 15 minutes. Let cool. Reduce the oven temperature to 275 degrees Fahrenheit.
4. Prepare the filling. In a stand mixer fitted with the paddle attachment, beat cream cheese, sugar, and salt on medium speed until soft and creamy, one to two minutes. Add ricotta and beat until the mixture is smooth and light, one to two minutes. Slowly add zest, cornstarch, and vanilla, and beat until combined. Beat in the eggs, one at a time. Do not overbeat the mixture once the eggs have been added. Fold in the chocolate by hand.
5. Pour the filling into the crust and smooth the top with a spatula. Bake until the edges look set, about one hour. Turn the oven off and let cheesecake cool in the oven with the door slightly ajar for one hour. Transfer to a rack and cool to room temperature, about three hours.
6. Refrigerate until thoroughly chilled, at least four hours.
7. Run a thin knife around the edges of the cheesecake again before removing from the pan. To slice, heat the knife under hot water between each cut and wipe it dry.