Add the sugar, sake, mirin, and soy sauce to a small skillet over medium heat. Let come to a bubble, and when the sauce boils turn the heat off. The sauce will thicken slightly.
For the chicken, preheat a skillet to medium-high heat with a drizzle of oil. Sear the chicken on both sides for 2 minutes each, then pour 2 tablespoons of the sauce into the skillet. Toss the sauce with the chicken and cook until the sauce coats the chicken and the chicken is cooked through.
Serve as desired with extra sauce for drizzling and dipping.
Notes
This sauce is delicious on any protein that you enjoy - make a double batch and it'll be good in the fridge for several weeks.
You may also grill your chicken or protein of choice. Simply grill the chicken as usual, brushing the sauce over the meat beginning halfway through cooking.