In a large bowl, whisk together the ricotta cheese, powdered sugar, vanilla extract, and cinnamon until smooth and well combined.
Gently fold in the mini chocolate chips and chopped pistachios, being careful not to overmix.
Transfer the filling to a piping bag or large zip-top bag with one corner cut off for easy filling.
Just before serving, pipe the ricotta mixture into both ends of each cannoli shell, filling them completely.
Dip the ends of filled cannoli in additional chopped pistachios if desired.
Dust with powdered sugar and serve immediately for the crispiest shells.
Notes
Storage tip: Keep unfilled shells in an airtight container for up to 1 week. Filling can be made 3 days ahead and refrigerated. Only fill shells 2-3 hours before serving to prevent sogginess. For best results, serve immediately after filling.