Pop popcorn according to package directions. Open bag carefully and pour into large bowl; discard unpopped kernels. In small saucepan over medium heat, heat corn syrup 3 minutes or just until boiling. Remove from heat. Stir in peanut butter and chocolate pieces until smooth. Pour syrup mixture over popcorn; toss to coat evenly. Cool about 8 minutes. Using heaping tablespoon, spoon out mixture onto a lightly-buttered wax paper-lined baking sheet. Flatten each popcorn mound slightly and make indentation in center with thumb or wooden spoon handle. Fill each center with topping if desired. Store in tightly covered container.