Remove and discard unpopped kernels from popped popcorn. In 3-quart saucepan, heat corn syrup, butter and peanut butter chips over medium heat 3 to 4 minutes, stirring frequently, until chips are melted and mixture is smooth. Stir in popcorn (mixture will be thick). Spoon over marshmallows; spread evenly to just barely cover marshmallows. Cool 30 minutes.