Wash sweet potatoes and cut into about 0.8 inch (2cm) thickness.
Peel the purple skin thickly and soak them in a large bowl of water to remove astringent taste for about 15-20 minutes.
Separate the chestnuts and syrup and set aside.
Drain the soaking water and place them into a pot with enough water to cover the sweet potatoes.
Crash the gardenia fruit and put it in a tea bag, (which you can get Japanese one coin shop such as Daiso).
Place the bag of crashed gardenia fruit or 1 tsp turmeric into the pot.
Bring the water to boil over high heat, then turn it down to medium to simmer for about 20 minutes. Insert a skewer to test the potato is cooked.
When the sweet potato is cooked, remove the bag of gardenia fruit, drain the water and while the sweet potato is hot, strain the potato using a sieve with a wooden spatula.
Place the strained sweet potato back into the cooking pot, with sugar, chestnuts syrup, mirin, and salt, cook over medium heat.
Stir the mashed sweet potato continually avoiding the bottom of the pot to burn.
When you can see the bottom of the pot, and the mashed sweet potatoes thickened slightly, turn the heat off.
Add the chestnuts to the pot and coat them with mashed sweet potatoes.
Leave the Kuri Kinton to cool down, then serve!
*1 weighing about 1 lb (500g) and when peeled 12oz (350g).*2 Contains about 12 candied chestnuts. *3 If you can not access this ingredient, substitute with 1 tsp turmeric like I did. *4 Total cooking time does not include soaking sweet potatoes in water.