In a large mixing bowl, stir together mascarpone cheese, pumpkin puree, powdered sugar, ricotta, and pumpkin pie spice until blended. Fold in 1/4 cup of the pumpkin seeds and the mini chocolate chips,then set aside. In a chilled mixing bowl, whip cream until stiff peaks form. Fold into pumpkin mixture. Cover and chill for 1-2 hours. Right before serving, pipe pumpkin filling into cannoli shells, filling all the way to the ends. Sprinkle the ends with remaining chopped pumpkin seeds. Sprinkle shell with powdered sugar and serve.