Korean Plum Extract or Green Plum Syrup has only appeared in Korean cooking in the last 10 years or so while Korean green plums have been used as far back as 200 AD.
Wash plums and let drain and dry completely. Remove stems.
Discard rotten plums. Sanitize the glass jar by rinsing it with boiling water or alcohol.
Lay sugar and plums alternately in the jar. Cover the jar and store in a cool, dry place for 90 days.