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According to my experience (and KM), this is one of the hardest type of 和菓子 to get right.
It's a steamed sweet that's wrapped in a skin called すり芋, made from, surprisingly, 山芋. I tried it with とろろ, which you can buy frozen in Yumi Hana, for example.
すり芋
Mix grated yam with twice its weight in sugar, then weigh that mixture.
Add 40% of that weight in non-glutinous rice flour (上用粉, which is not quite the same as 上新粉).
The ratio between the 中あん and the すり芋 should be 10:6 (so 30 gr 中あん, for example, to 18 gr skin).
I used 上新粉 to dust the dough and hands because it's so sticky, but it seems that potato starch is better here.
淡交テクスト recipe
15 gr boiled chestnut, 15 gr smooth あん, 18 gr すり芋.
Spray with 2:1 vinegar:water, place on baking paper in a steamer and steam on high heat for 8', then fan to cool.
My ideas
I think the chestnut is not a good match here, and I'd rather use candied walnuts and a bit of 味噌 in the 中あん
I also think they are a bit big!
KM suggested to use twice the amount of とろろ
Other ideas are: