First, clean and rinse the rice. Combine both rices in a large bowl and wash with cold water until the water runs clear.
Combine the rices, water, sake, mirin and soy sauce in a large pan. Bring to the boil then cover and reduce to a simmer, until the rice absorbs the liquid, about 20 mins.
Take the rice off the heat. The add in the chestnuts and cover with a lid, heating them through for 10 mins. Serve into bowls, sprinkle with black sesame seeds and enjoy.