Bring soy sauce, sugar, and mirin to a boil in a small saucepan over medium-high heat, stirring to dissolve sugar. Remove from heat and let teriyaki sauce cool completely.
Combine teryaiki sauce, ginger powder, and garlic in a bowl and stir until well mixed.
place chicken in a 1-gallon zipper-lock bag and add teriyaki sauce-ginger mixture. Press out air, seal bag, and turn to coat chicken with marinage. Refrigerate for at least 1 hour or up to 24 hours.
Turn on your BBQ to medium-high heat for 10-15 minutes. Clean and oil the cooking grate. Place chicken on the grill and cook until chicken is lightly charred and registers at least 175 degrees (about 6 minutes per side). Transfer to a cutting board and let rest for 10 minutes. Slice crosswise 1/2 inch thick and serve.