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1
Soak the red beans in plenty of water for at least 2 to 3 hours and ideally overnight, making sure beans are submerged fully by at least 1 to 2 inches of water. Rinse and drain.
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2
Next, combine the beans with 3 more cups of water in a pot and bring it to a boil over high heat. Once the water begins to boil, reduce the heat to low and let the beans simmer for about 1 1/2 to 2 hours. As the water boils down, you will need to add just enough water to keep the beans submerged. (For me, I added 1/2 cup after about 30 minutes of boiling, then another 1/2 cup every 15 to 20 minutes until the beans were done.) There is no need to stir the beans -- just keep them submerged in water. (Note: If you're concerned about any dirt or imperfections on the beans, blanch them before this step by bringing the water and beans to a boil, immediately draining and rinsing the beans, then proceeding with Step 2.)
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3
After an hour, test a bean by mashing it with a spoon or your fingers. If it splits as two halves, keep cooking. If it smushes easily, it's done simmering.
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4
Drain the beans, then add them to a food processor along with the sugar, and process on high until smooth. (If you don't have a food processor or blender, just drain the water, add the sugar directly to the pot, and keep cooking over low heat, mashing as you would mashed potatoes. It should be just fine that way, too.)
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5
Pour the paste into the pan and add the fat of your choice. If using the paste for steamed buns or mochi, opt for 1 to 2 tablespoons of vegetable oil. For mooncakes, use a solid fat: I used coconut oil, but many traditional mooncake fillings use lard. Stir until the fat is incorporated, then cook over medium heat for 3 to 5 minutes, stirring consistently, until the paste becomes glossy and forms a soft dough that sticks to the spoon in one mass.
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6
Remove from heat and chill until ready to use.
