Heat sesame oil in a pot over medium heat. Add the kabocha pieces and sauté for 2-3 minutes to lightly coat with oil. Pour in the dashi stock and bring to a boil. Add soy sauce, mirin, sake, sugar, and a pinch of salt. Reduce the heat to low, cover, and simmer for about 15 minutes until the kabocha is tender. Uncover and cook for another 2-3 minutes to allow the sauce to slightly reduce. Serve hot, garnished with sesame seeds if desired.