Melt 5 cups marshmallows, the corn syrup, butter and chocolate chips in the microwave for about a minute or two, stirring frequently. Stir until melted and add in vanilla. Pour over cereal in a large bowl. Mix quickly until completely coated with chocolate. Stir in 1 cup marshmallows. Press mixture evenly in 9 x 13 pan with buttered back of spoon.
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