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.