To make the teriyaki sauce, combine the soy sauce, brown sugar, garlic powder, ginger powder, and mirin in a saucepan and mix well. In a small bowl, mix the cornstarch and water to make a slurry and add it into the soy sauce mixture. Cook the sauce over medium heat while stirring occasionally until thicker in consistency. If the sauce becomes too thick, add some hot water. Remove from heat and sauce aside.
To make the chicken, debone the chicken thighs if needed. Season both sides with salt and pepper. Heat oil in a cooking pan at medium high heat and cook chicken thighs for 6-7 minutes on each side until fully cooked, depending on the thickness of your chicken, and until the skin is crispy. Remove from heat and place in bowl filled with rice. Pour the teriyaki sauce evenly on the chicken and top with sesame seeds and chopped green onions.