Wash the azuki beans and place in a large pot filled with about 4-5 cups of. Bring to a boil over high heat. Reduce the heat to low, cover the pot, and cook the azuki beans until soft, about 1 to 1 ½ hours. There will still be a considerable amount of liquid at this point.
Place a sieve over a bowl. Pour the beans and liquid into the strainer. Using a wooden spoon, spatula, or the palm of your hands, mash the beans through the sieve so that the bean skins stay in the sieve.
Pour the mashed beans and simmering liquid into a cheesecloth, cotton bag, or cotton towel and squeeze out all the water.
Put the unsweetened bean paste in a saucepan, add the sugar, and cook over low heat, stirring gently until the azuki bean paste is heated through and glossy. Season with a pinch of salt and mix well. The paste should have the consistency of mashed potatoes.
Notes
makes about 2 cups (more than you will need for the 3 crepes)