In a bowl, gently stir together the peanut butter, Cool Whip, and powdered sugar until well blended. Pour into pie shell. Top with chunks of Reese’s Peanut Butter Cups, if desired. Freeze for 8 hours or overnight. Then, just bring it right out of the freezer and slice it up. It will thaw pretty quickly right on the plate, but it’s meant to be eaten as sort of an ice cream pie! Be prepared to get hooked!