I try not to cover too many freshly produced sweets that can only be purchased if you actually live in or visit Tokyo. However, this is an exception because it is the first bean cake in Japan that I actually loved. It is, by far, the most accessible Japanese sweet for Western palates while maintaining a completely Japanese nature. Maybe everyone won't adore it as I do, but there's almost zero chance that a set of fully Westernized taste buds will dislike it.
The box of 12 as it is given to us.
Koganei Imo means "golden (colored) potato", and while that is what these are called, they are not made with potato. They are simply fashioned to resemble one. The actual cake is finely mashed white beans mixed with sugar and egg yolk then wrapped in a tender crust and liberally rolled in cinnamon. It is the cinnamon that gives these the real edge over any other bean cake I have tried, but they are also incredible because they are hand made and fresh everyday.
The box under the paper wrapper with silk-screened (by hand, very likely) paper. This is classic Japanese gift wrapping with a layer of wrapping under a top layer.
The interior of the cakes is soft and sweet and the aroma of cinnamon wafts at you the minute you start to peel back the paper on in. If you warm one just a bit in the microwave (don't overdo it), they are a slice of cinnamon heaven which carry all of the appeal of a cinnamon bun without the calories or sugary overload. The beans themselves lend only texture and not taste so there's no need to be squeamish about them because they're made with beans. The filling carries mostly the sugar flavor with the richness of the egg yolks.
A whole cake with it's wrapper covered with cinnamon. It's supposed to look like a potato. You can judge what it looks like.
You can buy these in Ningyocho in Tokyo at a shop called Kotobukido. There's a good write up on the shop with address and phone number at Tokyo Food Life and another write-up at Food Sake Tokyo. They are a little expensive, but well worth the cost. My box of 12 cost 1250 yen ($15.15). Each is about 6-7 cm (2.4 -2.7 inches) long and makes for a modest treat. You can buy them individually in paper packets or in souvenir size gift boxes. The shop will serve you hot tea in the winter and cold in the summer when you enter, and you can eat your bean cake there or take it with you.
If you visit Tokyo, I strongly recommend seeking out this shop and sampling this unique and approachable sweet. I go there less often than I'd like, but when I do I buy a dozen, immediately wrap their paper wrappers with plastic wrap and freeze them for long-term enjoyment. They don't suffer for it, and 20-25 seconds in the microwave takes one from frozen log to cinnamony ambrosia.