1Spray 13x9-inch pan with cooking spray.
2In large microwavable bowl, microwave butter uncovered on High 30 to 45 seconds or until melted. Add marshmallows; toss to coat. Microwave on High 1 minute 15 seconds to 1 minute 20 seconds, stirring every 30 seconds, until mixture can be stirred smooth. Stir in cereal until well coated. Press evenly into pan.
3In small bowl, microwave semisweet chocolate chips with 1/2 teaspoon oil uncovered on High 45 to 60 seconds, stirring every 15 seconds, until smooth. Repeat with white baking chips and remaining 1/2 teaspoon oil. Drizzle on bars. Refrigerate about 30 minutes or cool at room temperature until chocolate is set. For bars, cut into 6 rows by 4 rows.