1Butter bottom and sides of 13x9-inch pan. Remove and discard unpopped kernels from popped popcorn. In large bowl, mix popcorn, cereal and pretzels; set aside.
2In 2-cup microwavable measuring cup, microwave brown sugar, corn syrup and butter uncovered on High 1 to 2 minutes or until boiling, stirring after 1 minute. Microwave 30 seconds longer; stir. Pour over cereal mixture in bowl, stirring until evenly coated. Press firmly in pan.
3In small microwavable bowl, microwave chocolate chips uncovered on High 30 seconds to 1 minute or until chips can be stirred smooth. Drizzle over bars. Cool completely, about 30 minutes. For bars, cut into 8 rows by 4 rows.