Measure cereal out into a large bowl and set aside till later. Grease a 9×13″ pan with 1 Tbsp softened butter.
Heat corn syrup, remaining 2 Tbsp butter, and chocolate chips in a saucepan over medium-high heat until just boiling, stirring constantly. Remove from heat and stir in vanilla extract.
Pour chocolate mixture over cereal, stirring until cereal is evenly coated. Fold in marshmallows.
Press mixture evenly into pan. Let stand until firm or place in refrigerator until firm. Cut into squares. Store in an airtight container in a cool place.
