Rinse the adzuki beans under cold water. Place them in a large bowl and cover with plenty of cold water. Soak for at least 8 hours or overnight.

Drain the soaked beans and transfer to a large pot. Cover with fresh water. Bring to a boil, then reduce heat to a simmer and cook for 60-90 minutes, or until beans are very soft and crush easily.

Drain any remaining water. For chunky paste (Tsubuan), mash the beans in the pot. For smooth paste (Koshian), press the beans through a fine-mesh sieve to remove the skins.

Return the bean paste (chunky or smooth) to the pot. Add the sugar and salt. Cook over low-medium heat, stirring constantly for 10-15 minutes until the paste thickens. It will firm up more as it cools.

Remove from heat and let it cool completely. Store in an airtight container in the refrigerator for up to a week or freeze for up to 3 months.