Cut the chicken thighs into bite-sized pieces, about 1-inch cubes.
Place the cut chicken pieces into a pan over medium heat. To the pan, add soy sauce, mirin, grated ginger, sugar, and garlic powder.
Cook the chicken for 25 minutes while stirring occasionally. The initially watery sauce will reduce and thicken. Cook until the Teriyaki sauce is bubbly and sticky and the chicken is well-done.
Garnish the Chicken Teriyaki with chopped scallions and a sprinkle of sesame seeds to add color and flavor. Serve with a side of rice and vegetables.