In a large saucepan, combine marshmallows and butter. Cook and stir over medium heat until marshmallows are melted. Stir in cereal. With greased hands, press onto the bottom and up the sides of a greased 9-in. pie plate; set aside., In a large bowl, dissolve gelatin in boiling water; stir in cold water. Refrigerate until partially set, about 1 hour. , In a small bowl, beat cream until it begins to thicken. Add confectioners' sugar; beat until soft peaks form., Fold berries and 3 cups whipped cream into gelatin mixture. Pour into crust. Refrigerate pie and remaining whipped cream for up to 4 hours. Garnish with reserved cream and additional blueberries.