Beat together avocado and ricotta. Sift confectioner’s sugar into mixture; mix until smooth. Reserve tightly covered in refrigerator up to 24 hours. Remove filling from refrigerator 30 minutes before serving. Beat again, if necessary. With a 1/2 inch plain tube, pipe filling into each end of the cannoli shell. Garnish each end with chopped pistachios; dust top of shell with confectioner’s sugar.