I found this recipe on the back of a cereal box probably 20 years ago, tucked it my recipe box and forgot about it. I pulled it out last year and made them to take on a camping trip and they were a huge hit! I've made them several times since then and they don't stick around long!
Combine margarine, corn syrup and chocolate chips in a 3 quart saucepan. Heat to boiling, stirring constantly until smooth. Remove from heat. Stir in vanilla.
Pour hot chocolate mixture over cereal and quickly toss to coat. Fold in marshmallows one cup at a time. Press mixture into prepared pan with the back of a spoon coated with butter or cooking spray. A few extra marshmallows can be added to fill in nooks and crannys on the top.
Let stand until firm; at least an hour. Cut into bars or squares. Store covered at room temperature.