Prepare Red Beans: Soak 1 cup of dried red beans for 10 hours. Strain and cook with 4 cups of cold water in a thick pan for 1 hour over medium heat.
Make Dough: Melt 2 tablespoons of unsalted butter in a pot. Add 2 tablespoons of sugar, then turn off the heat. Add 1 and 1/2 cups of milk and 2 teaspoons of active dry yeast. Let it stand for 5 minutes. Add 1 teaspoon of salt and 3 and 1/2 cups of all-purpose flour. Stir with a wooden spoon and knead until the dough is slightly sticky. Let it rise for 1 hour at room temperature.
Prepare Red Bean Filling: After the beans are cooked, add 1/4 teaspoon of salt, 1/4 cup of organic dark brown sugar, 1/4 cup of rice syrup, and 2 teaspoons of vanilla extract. Smash the beans slightly with a potato masher to a chunky consistency.
Shape and Fill Buns: After the dough has risen, divide it into ten pieces. Flatten each piece and fill with the red bean paste. Seal the buns and prepare them for steaming.
Steam Buns: Steam the buns in a steamer for an appropriate amount of time until they are fluffy and cooked through.