Butter 9x13 pan. Heat syrup, butter and chocolate chips to boiling, stirring constantly. Remove from heat; stir in vanilla. Pour over cereal in bowl; toss until coated. Fold in marshmallows, 1 c at a time. Press in pan with buttered back of spoon. Let stand 1 hour. Cut into 2-inch squares. Store loosely covered at room temperature up to 2 days.
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