Preheat oven to 400° and spray a casserole dish with non-stick spray. Put cubed sweet potatoes in the dish.
In a separate bowl, mix the sesame oil, brown sugar, garlic, and soy sauce together until the sugar is dissolved as much as possible. Pour the sauce over the sweet potato cubes and stir until well coated on all sides. Cover the dish with foil (or a lid if you have one) and place in the oven for 20 minutes.
Uncover, stir and return to the oven. Bake for another 30 minutes, stirring every 10 minutes. Add the sesame seeds the last time you stir (10 min. left to bake).