Wash sweet potatoes and peel the purple skin thickly.
Cut the potatoes into about 0.8 inch (2cm) thickness and soak them in a large bowl of water for about 15-20 minutes to remove the astringent taste.
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.
Crush the gardenia fruit and put it in a tea bag (which you can get at Japanese one coin shop such as Daiso).
Place the bag of crushed 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 if 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 and cook over medium heat.
Stir the mashed sweet potato continually avoiding the bottom of the pot burning.
When the mixture has 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!
Notes
*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.