Add ½ cup mirin, ⅓ cup soy sauce, and ⅓ cup sake to a pot and bring the mixture to a boil. If you are using a gas stove, ensure your ventilation is running and be cautious not to let the flame rise up the sides of the pot, as this may ignite the alcohol vapors. If that happens, don’t panic; just cover the pot with a lid to extinguish the flames. Continue boiling the liquid until it no longer smells like alcohol (about 2-3 minutes). Let the brine cool to room temperature.
Prepare 4 skin-on chicken breasts by trimming off excess fat and any cartilage or tendon and place them in a sealable bag.
When the brine has cooled to room temperature, pour it into the bag with the chicken and seal the bag, pressing out as much air as you can. Cure the chicken in the refrigerator for 2-4 days, flipping it over once a day to ensure even brining.
When the chicken is done curing, fill a water bath and set your sous vide machine to 143°F (62°C)
Drain the brine from the chicken and use paper towels to dry the surface of the meat off.
Place each breast into a heat-safe vacuum bag and vacuum seal each one. To give the chicken a round shape, I like to roll the bag around the chicken to form a cylinder and tape the end shut.
Place the bags of chicken in your preheated sous vide setup and cook for 2 ½ to 3 hours.
When the timer is up, you can slice and serve the chicken chashu hot, or you can rapidly chill it in a cold water bath and keep the bagged chicken in the fridge for up to a week.