In a medium saucepan, mix together the pumpkin, marshmallows and cinnamon over medium low heat. Stir continuously until marshmallows are completely melted. Transfer to a large bowl and place in the refrigerator for around 1 hour.
Once the mixture is cooled, whip the heavy cream in a stand mixer until soft peaks form. Set aside.
Beat the pumpkin mixture until slightly fluffy, around 1 minutes. Fold in the whipped cream mixture, then spoon into the prepared crust.
Place in the fridge and allow to chill for around 2 hours, or until firm.