Cut chicken into about 2" (5cm) pieces. Remove moisture with paper towels and sprinkle salt, garlic powder, and pepper. Coat the surface of the chicken with flour. In a small bowl, mix soy sauce, mirin, wasabi paste, and set aside.
Heat a large frying pan at medium high heat with oil. Brown the chicken on both sides, reduce the heat to medium, and cook for 10 minutes or until cooked through, flipping once or twice. Add the sauce, remove from the heat, and coat the chicken with the sauce. Serve on a plate and add a bit more wasabi if you like.