Place shio koji and sugar in a plastic bag, knead to mix, then add the chicken breast. Massage until the chicken is fully coated. Remove air from the bag, seal it, and refrigerate overnight or for one day.

Fill a pot with enough water to fully submerge the chicken. Add the marinated chicken and heat over medium-low heat. Gently bring to a simmer, skimming off any foam. Cook on low heat for 3 minutes, then turn off the heat. Cover the pot and let it sit for 50–60 minutes.