Pat dry pork belly then lay it skin side down on a clean cutting board. Season lightly with salt and pepper.
1 lb pork belly, salt and pepper
Neatly roll the meat into a roll then secure it tightly with cooking twine or butcher strings.
In a medium cooking pot, add cooking sake and enough water to submerge roll of pork belly completely.
6 c water, 1 c cooking sake
Bring to boil then reduce the heat to medium-high and simmer until the liquid reduce more than half, approximately 30 to 40 minutes.
Skim and discard any impurity floating to the surface.
Add mirin, brown sugar, and soy sauce to the pot. Bring the pot back to boil until sugar dissolved completely.
¼ c koikuchi shoyu, 3 tbsp sweet mirin, 2 tbsp light brown sugar
Remember to turn the heat occasionally so it can cook evenly in the sauce.
You can partially cover the pot to speed up the process. Keep simmer the pot until the sauce reduce rapidly and start bubbling on the surface.
When the sauce becomes caramelized and the pork belly looks shiny dark brown, the chasu is done.
Turn off the heat. Take the pork belly out and cool down to room temperature before slicing into thin slices. Preserve the sauce for later use.