1. The night before place the ricotta in a yogurt strainer or cheesecloth or paper-towel lined strainer over a bowl. Drain overnight in the refrigerator. Discard the whey.
2. Preheat the oven to 400°F. Lightly spray 12 cannoli tubes with spray oil. Starting with the corner of one wonton wrapper in the center, roll the wrapper around the tube. Glue the opposite corner to the tube with a dab of cornstarch paste. Lightly spray the outside of the cannoli with oil. Roll the remaining wrappers the same way.
3. Bake the cannoli until golden brown and crisp, about 4 to 6 minutes. Let cool slightly, then slide the pastry shells off the tubes onto a cake rack and let cool to room temperature. Brush with the melted butter.
4. Prepare the filling. In a large bowl combine the drained ricotta, sugar, orange flower water, vanilla, and lemon zest and whisk to mix. Correct the sweetness, adding sugar or orange flower water to taste. Transfer the filling to a piping bag fitted with a 1/2 inch star tip. The recipe can be prepared several hours ahead to this stage.
5. Just before serving, pipe the ricotta mixture into the pastry shells, adding filling from each end. As you withdraw the piping tube, make a rosette at each end. Sprinkle the ends with chopped pistachios and serve at once.