In a bowl, mix together soy sauce, balsamic vinegar, ketchup, brown sugar, garlic powder and chili powder. Add chicken legs and marinate for 24 hours. Preheat oven at 200°c. Place chicken legs (skin side up) on a lined baking sheet and bake for 30 minutes or until the chicken is fully cooked. Whilst chicken is cooking, remove from oven every 10 mins and baste remaining marinade over the chicken for more depth of flavour. Garnish with sliced spring onions and sesame seeds, and serve immediately.