In a large microwave-safe bowl, combine the pumpkin, marshmallows and chips. Microwave, uncovered, on high for 1 minute; stir. Microwave 20-30 seconds longer or until marshmallows are melted and mixture is smooth, stirring every 15 seconds. Cool to room temperature, stirring several times.
Fold whipped topping into pumpkin mixture. Spoon into crust. Garish with the additional topping and miniature chips if desired. Refrigerate for at least 2 hours before slicing.