sweetened red beans
Small red beans cooked soft and thickened with sugar, left whole or mashed to a paste. Sweet, gently nutty, creamy.
Sweetened red beans are small, dark red adzuki beans cooked until soft and then simmered down with sugar into a thick, spreadable mass. In Korea they are used almost entirely as a sweet ingredient, for instance as a filling for pastries and rice cakes or as a topping on shaved ice. They taste sweet with a mild, slightly nutty and earthy bean note; they come coarse, with whole beans still visible, or sieved smooth, and which one suits depends on whether the filling should stay chunky or spread evenly. Ready-sweetened beans are sold in cans or pouches at Asian grocers, and dried adzuki beans for cooking your own are available there as well as in health food and organic shops.
Substitute:
The best option is to cook them yourself from dried adzuki beans with sugar and a pinch of salt; it simply takes longer. Other beans are not a good substitute: kidney beans have thicker skins and a more savoury flavour, and the result turns mealy rather than creamy.