Place corn syrup, butter, and chocolate chips in a medium, clear microwavable bowl. Microwave uncovered on medium-high (70%) 1 minute 30 seconds. Stir. Microwave uncovered about 2 minutes 30 seconds longer or until large bubbles form on the surface. Stir in vanilla.

Pour the chocolate mixture over the cereal. Stir until the cereal is evenly coated. Stir in marshmallows, 1 cup at a time. Rub butter on back of a spoon. Use the spoon to press the cereal mixture into the pan. Let stand 1 hour or until firm.

Cut into 2-inch squares. Store loosely covered at room temperature for no longer than 2 days. 24 treats.