Wash the Adzuki beans and pick out any that look bad or rotten.
Pour beans into a large pot. Then pour-in enough water to cover 1~1.5 inches above the beans. Bring the pot up to boil on a high heat. Once it's boiling, place the lid on. Then boil on high heat for 10 minutes.
After 10 minutes, drain the water out through a sieve. Place the beans back into the pot. Refill the pot with fresh water - up to 1~1.5 inches above the beans. Place on high heat and bring back to a boil.
Once its boiling, reduce the heat to a low. Let it gently boil on low heat - with the lid on - for 1 hour. (Note: Beans soak up alot of water! Check on your pot during the hour and pour-in more water. It may require 1-2 refills.)
After 1 hour, the beans should be very soft. You should be able to smoosh one very easily with your fingers. If not, let it boil for longer.
Drain the water once more through a sieve. Place the boiled beans back into the pot.
Now pour the sugar (1 cup) into the pot. Mix the two evenly. (Note: You can add more sugar if you'd like based on your sweetness preference.)
Place the pot back onto a low heat. We want the residual water from the beans to dissipate. As the liquid starts to boil again and reduce, use a spatula and mix the beans around. This will break the beans and turn it into a paste.
When you're able to scrape the bottom of the pot and the paste doesn't move back quickly, you're done (reference video). Turn off the heat.
Give a few shakes of salt over the beans - this accentuates the sweetness. Then stir-once more.
Scoop and spread the bean paste onto a pan. Let it cool down completely.
Store leftovers in the freezer.