Clean chicken thighs by removing tendons and excess fat. Rinse under running water. For frozen chicken, thaw and soak in milk or marinate with mirin, pepper, and ginger powder to remove odors.

Place chicken thighs, soy sauce, ketchup, vinegar, sugar, butter, mirin, and water into a pan.

Cook on high heat until the sauce starts boiling. Sprinkle a pinch of pepper.

Once butter melts, reduce to low heat and cover the pan. Let it simmer.

Simmer until the sauce thickens to a desired consistency. Stop cooking if you prefer a more moist texture.