Prepare the Sauce:
In a small bowl, mix soy sauce, mirin, sake, sugar, garlic, and ginger. Whisk until sugar is fully dissolved.
Marinate the Chicken:
Place the chicken thighs in a shallow dish and pour half of the sauce over them. Cover and let it marinate for 15 minutes.
Cook the Chicken:
Heat a non-stick skillet over medium heat. Add the marinated chicken thighs, skin-side down (if applicable). Sear for 3-4 minutes until golden brown.
Flip and Simmer:
Flip the chicken thighs, reduce the heat to low, and pour the remaining sauce into the skillet. Cover and let it cook for 8-10 minutes, flipping occasionally.
Thicken the Sauce (Optional):
If you prefer a thicker sauce, dissolve cornstarch in water and stir it into the skillet. Let it simmer for 2-3 minutes until the sauce thickens.
Slice and Serve:
Remove the chicken from the skillet, slice into strips, and serve over steamed rice. Drizzle the sauce on top.